Overview of the Yukon River

Exerpts taken from:

Environmental and Hydrologic Overview of the Yukon River Basin, Alaska and Canada
By Timothy P. Brabets, Bronwen Wang, and Robert H. Meade
U.S. GEOLOGICAL SURVEY
Water-Resources Investigations Report 99-4204

SUMMARY

Introduction

Few rivers have the intrinsic allure of the Yukon River. Its history, people, and mystique have innate appeal. The Yukon River is a transportation corridor in a vast area of roadless Alaska. Salmon species migrate the entire length of the river to spawn and are also a staple of the subsistence lifestyle of rural villages. Villages and towns obtain water for drinking from the river and associated aquifers. Recreational activities abound for both residents and tourists.

The Yukon River Basin (figs. 1 and 2) is located in northwestern Canada and central Alaska, and is approximately 330,000 square miles in area. The basin represents one of the largest and most diverse ecosystems in North America. Twenty ecoregions compose the Yukon River Basin, which indicates the large diversity of natural features of the watershed, such as climate, soils, permafrost, and geology.

Although the annual mean discharge of the Yukon River near its mouth is more than 200,000 cubic feet per second, most of the flow occurs in the summer months from snowmelt, rainfall, and glacial melt. Eight major rivers flow into the Yukon River. Two of these rivers, the Tanana River and the White River, are glacier-fed rivers and together account for 29 percent of the total water flow of the Yukon. Two others, the Porcupine River and the Koyukuk River, are underlain by continuous permafrost and drain larger areas than the Tanana and the White, but together contribute only 22 percent of the total water flow in the Yukon.

Despite its remoteness and perceived invulnerability, the Yukon River Basin is changing. For example, from 1991 to 1999, the number of growing days in Alaska for four ecoregions has ranged from 130 to 194 days. Air temperature records from 1961-90 indicate a warming trend on the order of 1.4 °F (0.75 °C) per decade at latitudes where the Yukon River is located (fig. 4) (Chapman and Walsh, 1993). If this warming trend continues, the growing season will likely increase. Climate changes will also influence the permafrost distribution, glacial runoff, and biogeochemical fluxes within and from the basin (BESIS, 1997). The Yukon River is also fundamental to the Bering Sea ecosystem (fig. 1), providing most of the fresh water runoff, sediments, and dissolved solutes in the eastern part of the sea (Lisitsysn, 1969). Thus, processes that influence the Yukon River could in turn influence the Bering Sea.

Description and History of the Yukon River Basin

The Yukon River and its Major Tributaries, Exploration of the Yukon River Basin, People and Land, Economic Activity

The Yukon River and its Major Tributaries

The Yukon River Basin is the fourth largest basin in North America and the fifth largest in terms of average discharge (Schumm and Winkley, 1994). Although no universal agreement exists as to the source of the Yukon River, it is believed to originate from the Llewellyn Glacier, near Atlin Lake, in northwestern British Columbia (Parfit, 1998) (fig. 5A). From this point, the river flows for more than 2,000 miles in a broad arc through the Yukon Territory of Canada and central Alaska, emptying into the Bering Sea.

From its headwaters, the Yukon River generally flows northwestward to the Canada /Alaska boundary. Near the outlet of Lake Laberge (fig. 5A) above Frank Creek, the river is approximately 300 ft wide (fig. 6A), but downstream from the junction of the Teslin River at Carmacks, the width increases to about 600 ft (fig. 6B). Below Carmacks, the Dawson Range lies on the west and the Ogilvie and Pelly Mountains lie on the east. The Pelly and Stewart Rivers (fig. 5A), whose sources are along the Yukon Territory/Northwest Territories border, drain about 38,000 square miles. The White River (fig. 5A) drains about 18,000 square miles, and includes the extensive snowfields and glaciers of the Wrangell–St. Elias Mountains. The inflow from these three large rivers increases the width of the Yukon to approximately 1,000 ft at Dawson (fig. 6C).

At Fortymile, in Canada, about 60 river miles upstream from Eagle (fig. 5B), the Yukon River flows between bluffs of the Tanana Uplands on the south and the Ogilvie Mountains on the north. At Eagle, the width of the Yukon has increased to about 1,500 ft (fig. 6D). From Eagle, the Yukon flows for about 150 miles through the Yukon-Charley Rivers National Preserve to Circle (fig. 5B). This national park encompasses parts of the valley on tributaries between Eagle and Circle.

At Circle, the altitude of the Yukon River is approximately 600 ft above sea level and yet the river is still more than 1,000 miles inland. Circle marks the beginning of the Yukon Flats, a large lowland area crisscrossed by meandering river channels that are constantly shifting. Much of the lowland is part of the Yukon Flats National Wildlife Refuge. The landscape is characterized by flat terrain and is encircled by mountains, which trap the heat and the cold. This feature results in extreme air temperatures in the summer (as high as 100F) and winter (as low as -30F).

Approximately 60 miles downstream from Circle is the village of Fort Yukon (fig. 5B), where the Porcupine River enters the Yukon River from the northeast (fig. 5A). The Porcupine River drains about 45,000 square miles of the northeast part of the Yukon River Basin and is about 500 miles long. Downstream from Fort Yukon, the Chandalar River, which drains the Brooks Range, enters the Yukon from the north. The Yukon River reaches its northernmost point at the Arctic Circle at this location, and begins to flow westward and southward to the Bering Sea.

Stevens Village (fig. 5B) marks the approximate end of the Yukon Flats. The Yukon River flows in a more confined area, sometimes referred to as Rampart Canyon. At this point, the river is approximately 2,000 ft wide (fig. 6E) and is the location where the trans-Alaska oil pipeline crosses the river. Approximately 150 miles downstream from Stevens Village, the Tanana River enters the Yukon from the southeast (fig. 5A). The Tanana River is a large tributary to the Yukon and drains approximately 44,000 square miles. Included in the Tanana drainage is the north side of the Alaska Range, an extensively glaciated area.

Past the Tanana River, the Nowitna National Wildlife Refuge is located on the south side of the Yukon River for approximately 70 miles. The Nowitna River, a National Wild and Scenic River, enters the Yukon from the southeast. Past the wildlife refuge is the village of Ruby, where the Melozitna River enters from the north (fig. 5A). The width of the Yukon is approximately 2,500 ft at Ruby (fig. 6F). Galena, a village located about 50 miles downstream from Ruby is the largest community in this part of the Yukon River Basin.

Downstream from Galena, the Koyukuk River, a major tributary of the Yukon River, enters the Yukon from the north. The Koyukuk, which drains much of the north-central part of the Yukon River Basin, has a drainage area of about 35,000 square miles and is about 400 miles in length. At this point, the Yukon changes direction, flowing almost due south for about 160 miles. The Innoko National Wildlife Refuge is located on the east side of the river and near the end of this stretch of the Yukon, the Innoko River enters the Yukon near the village of Holy Cross. The Innoko River drains much of the area between the Yukon and Kuskokwim Rivers and flows in a broad S for about 500 miles.

Holy Cross is located about 280 miles upstream from the mouth of the Yukon. Several miles downstream from this village, the Yukon River begins to flow to the west, between hills and bluffs approximately 2,000 ft high. At Ohogamiut, an abandoned fishing village, the river turns northward. As the Yukon passes Pilot Station, the channel is about 3,000 ft wide (fig. 6G). Near Saint Marys, the Andreafsky River enters. The Andreafsky River is relatively small but has been designated a National Wild and Scenic river. From Saint Marys, the Yukon River flows through many channels in a large wetland area and then to the Bering Sea.

Environmental Characterisics of the Yukon River Basin

Physiography, Climate, Geology, Land Cover, Soils, Permafrost, Ecoregions

Physiography

The purpose of physiographic classification is to divide an area into smaller regions that are topographically distinct from surrounding regions. Thus, the boundaries of the physiographic regions are typically drawn where the topography changes in character. Physiographic divisions of the Yukon River Basin for Alaska were classified by Wahrhaftig (1965) and for Canada by Bostock (1970). Five general physiographic regions are present in the Yukon River Basin (fig. 8):

• (1) rolling topography and gentle slopes, 37 percent;

• (2) low mountains, generally rolling, 24 percent;

• (3) plains and lowlands, 20 percent;

• (4) moderately high rugged mountains, 17 percent; and

• (5) extremely high rugged mountains, 2 percent.

The following specific descriptions are modified from Wahrhaftig (1965) and can be inferred from the digital elevation map of the Yukon River Basin (fig. 2).

Alaska Range (Central and Eastern Part)—The central and eastern part of the Alaska Range consists of two or three parallel rugged glaciated ridges, 6,000-9,000 ft in altitude, surmounted by groups of extremely rugged snow-capped mountains more than 9,500 ft in altitude. Mount McKinley, 20,320 ft high and the highest mountain in North America, is located in this part of the Alaska Range. Most of the rivers and streams flow into the Tanana River. Rivers are swift and braided, and most rivers head in glaciers. The high mountains are sheathed in ice, and valley glaciers as much as 40 miles long and 5 miles wide radiate from them.

Central and Eastern Brooks Range—The Central and Eastern Brooks Range is a wilderness of rugged glaciated easttrending ridges that rise to summits 7,000-8,000 ft in altitude in the northern part and 4,000-6,000 ft in altitude in the southern part. The eastern part of the range has belts of hard and soft sedimentary and volcanic rocks. The mountains have cliff-and-bench slopes characteristic of glacially eroded bedded rocks. Major rivers flow southward to the Yukon and Koyukuk Rivers in flat-floored glaciated valleys ranging from 0.5 to 2 miles in width. Small cirque glaciers are common in the higher parts of the range.

Indian River Uplands—This region consists of groups of low gentle ridges having rounded accordant summits at 1,500- 2,000 ft altitude interspersed with irregular lowlands and broad flat divides. A few mountains rise to 4,000 ft in altitude. The Koyukuk and Kanuti Rivers cross the upland in narrow canyons a few hundred feet deep. Most of the region is drained by the Koyukuk River and its tributaries. Numerous thaw lakes are in the lowlands, valleys, and broad passes. Although there are no glaciers, the entire land area is underlain by permafrost.

Innoko Lowlands—The Innoko Lowlands are a group of flat flood plains, dendritic in pattern, whose bounding slopes are generally steep banks cut into the surrounding hills. The Yukon River and a large tributary, the Innoko River, cross the lowlands. The main part of the lowlands has a complex intersecting network of meandering sloughs of these two streams. Oxbow and meanderscroll lakes are abundant in recently abandoned flood plains and partly silted sloughs. Thaw lakes also abound in old flood plains and on gentle silt-covered slopes.

Kokrine-Hodzana Highlands—This region consists of even-topped rounded ridges rising to 2,000-4,000 ft in altitude surmounted by isolated areas of more rugged mountains. A rugged compact highland in the northeastern part has many peaks between 4,500 and 5,700 ft in altitude. The irregular drainage divide between the Yukon River and its large tributary, the Koyukuk River, passes through these highlands. Drainage to the Yukon River is by way of the Hodzana, Tozitna, Melozitna, and Dall Rivers and many shorter streams. Drainage to the Koyukuk River is by the Kanuti River and the South Fork Koyukuk River.

Koyukuk Flats—The Koyukuk Flats form an extensive lowland of irregular outline at the junction of the Yukon and Koyukuk Rivers. The central part of the Koyukuk Flats are flat plains 5-20 miles wide, along the major rivers. The parts immediately adjacent to the rivers are meander belts 5-10 miles wide and the parts farther away are dotted by thaw lakes. Broad rolling silt plains stand 100-200 ft above these central plains and merge with the surrounding uplands. The Flats are drained by the Yukon River.

Northern Foothills of the Alaska Range—The topography of this region consists of flat-topped east-trending ridges 2,000- 4,500 ft in altitude, 3-7 miles wide, and 5-20 miles long that are separated by rolling lowlands 700-1,500 ft in altitude and 2-10 miles wide. All rivers and streams in this region flow into the Tanana River. No glaciers are present in this region.

Nowitna Lowland—The Nowitna Lowland is a rolling siltcovered tableland ranging from 250-900 ft in altitude and having a local relief of 50-250 ft and slopes of 100-150 ft/miles into which flat flood plains of the major rivers have been incised 150-300 ft. The entire lowland is drained by the Yukon River, which follows the northern boundary. The confluence of the Yukon River with the Tanana River is in the eastern part of the lowland. The southern part of the lowland is drained by the Nowitna River, a tributary of the Yukon River.

Nulato Hills—The Nulato Hills consist of northeast-trending even-crested ridges, 1,000-2,000 ft in altitude, having gentle slopes. Valleys are narrow and have flat floors that are generally trenched in their upstream parts to depths of about 30 ft. Streams flow to the Yukon River.

Ogilvie Mountains—The Ogilvie Mountains have steep slopes and deep narrow valleys. Mountain peaks rise to 5,000 ft in altitude, and local relief is as much as 4,000 ft. The ridges are interconnected and passes are few. The narrow valleys are interrupted by gorges where rivers cross cliff-forming layers of rock. The major river drainages are the Kandik, Nation, and Tatonduk Rivers, all tributaries of the Yukon River. No glaciers are present, but most of this region is underlain by permafrost.

Porcupine Plateau—The Porcupine Plateau is dominated by low ridges having gentle slopes and rounded to flat summits 1,500-2,500 ft in altitude. A few mountains rise to 3,500 ft. Valley floors are broad and valley patterns are irregular. The Chandalar, Sheenjek, and Coleen Rivers rise in the Brooks Range and flow southward across the plateau in broad valley floors with moraines and outwash terraces. The Porcupine River crosses the plateau in a narrow cliff-lined canyon 50-500 ft deep. The Black and Little Black Rivers, which drain the southeastern part of the area, meander through broad irregular flats. The Porcupine Plateau has no glaciers, but the entire area is underlain by continuous permafrost.

Tanana-Kuskokwim Lowland—This lowland is a broad depression bordering the Alaska Range on the north. The central and eastern parts of the lowland are drained by the Tanana River. Braided glacial streams rising in the Alaska Range flow northward across the lowland. Thaw lakes are present in areas of fine alluvium and the entire area consists of permafrost.

Tozitna-Melozitna Lowland—This long narrow rolling plain, 5-10 miles wide, is drained by the Tozitna and Melozitna Rivers. These two rivers flow southward from the lowland in narrow gorges across the Kokrine-Hodzana Highlands to the Yukon River. The lowland contains numerous thaw and oxbow lakes. Discontinuous areas of permafrost are present.

Wrangell–St. Elias Mountains—The Wrangell Mountains are an oval group of shield and composite volcanoes that rise above a low plain on the north and west and above heavily glaciated cliffed and castellated ridges on the south and east. About 25 percent of the region drains into the Tanana River by way of the Nabesna and Chisana Rivers and into the Yukon River by way of the White River. The St. Elias Mountains are probably the most spectacular mountains of North America. Massive isolated blocklike mountains 14,000-19,000 ft in altitude rise at intervals of 5-30 miles from a network of narrow ridges and sharp peaks. The average altitude of icefields in the interconnected valley system is 3,000- 7,000 ft. Local relief is extreme and jagged cliffs abound.

Yukon Flats—The Yukon Flats region consists of marshy lake-dotted flats rising from 300 ft in altitude on the west to 600- 900 ft on the north and east. The northern part of the flats is made up of gently sloping outwash fans of the Chandalar, Christian, and Sheenjek Rivers. The southeastern part of the flats is the broad gentle outwash fan of the Yukon River. Other areas are flat flood plains. Rolling silt and gravel-covered marginal terraces having sharp escarpments 150-600 ft high rise above the flats and slope gradually up to altitudes of about 1,500 ft at the base of surrounding uplands and mountains. The region is drained by the Yukon River, which has a braided course southeast of the bend at Fort Yukon and a meandering course, containing many sloughs, southwest
of the bend at Fort Yukon. Most tributaries rise in surrounding uplands and mountains and have meandering courses through the flats.

Yukon-Kuskokwim Coastal Lowland—The Yukon- Kuskokwim Coastal Lowland is a triangular lake-dotted marshy plain rising from sea level on its west margin to 100-300 ft at its east end. Low beach ridges, marked by lines of thaw lakes, lie along part of the west coast. The lowland is crossed by meandering streams of extremely low gradient, many of them distributaries or former channels of the Yukon River. The Yukon River flows along the base of hills on the north side of the lowland and is building a delta into the Bering Sea. This region is dotted with innumerable thaw lakes, many of them 10 or more miles long. Probably 30-50 percent of the lowland is lake surface.

Yukon-Tanana Upland—The Yukon-Tanana Upland is characterized by rounded even-topped ridges. In the western part, these rounded ridges trend northwestward to eastward and have altitudes of 1,500-3,000 ft. The ridges are surmounted by compact rugged mountains 4,000-5,000 ft in altitude. Ridges in the eastern part have no preferred direction, are 3,000-5,000 ft in altitude and rise 1,500-3,000 ft above adjacent valleys. Valleys in the western part are generally flat, alluvium floored, and 0.25-0.50 miles wide to within a few miles of headwaters. Streams in the eastern part that drain to the Yukon River flow in narrow V-shaped terraced canyons. Streams flow southward to the Tanana River or northward to the Yukon River. No glaciers are in the region, but the entire section is underlain by discontinuous permafrost.

Ecoregions

Ecoregions are areas with common ecological settings that have relatively homogeneous features such as natural vegetation, geology, mineral availability from soils, physiography, and land use and land cover (Omernik, 1995). The Yukon River Basin has been classified into 20 ecoregions (Gallant and others, 1995; Ecological Stratification Working Group, 1995) (table 4; fig. 16). The Interior Forested Lowlands and Uplands ecoregion and the Interior Highlands ecoregion are the most dominant ecoregions in the basin. Descriptions of the ecoregions follow and are taken from Gallant and others and the Ecological Stratification Working Group.

Alaska Range—The mountains of south-central Alaska, the Alaska Range, are very high and steep. This ecoregion is covered by rocky slopes, icefields, and glaciers. Much of the area is barren of vegetation. The Alaska Range has a continental climate, but because of the extreme height of many of the ridges and peaks, annual precipitation at higher altitudes is similar to that measured for some ecoregions having maritime climate. Climate is influenced by the mountains. Weather data for the region indicate that winter daily low temperatures average about - 12 °F and daily high temperatures about 27 °F at lower altitudes. Summer daily low temperatures average about 36 °F and daily high temperatures about 64 °F. Mean annual precipitation in lowlands is approximately 15 in. and snowfall ranges from 59 to 120 in. Average annual precipitation for the mountain peaks is estimated at 80 in. and snowfall is estimated at 400 in. The terrain of the ecoregion consists of steep, rugged mountain ridges separated by broad valleys. Altitudes are 2,000 ft in the lower valleys and commonly rise to greater than 13,000 ft on mountain peaks. Slope gradients, which are almost always greater than 5 degrees on hillslopes, exceed 25 degrees on some mountains. The part of the ecoregion contained in the Yukon River Basin is part of a broad syncline having Cretaceous rocks in the center and Paleozoic and Precambrian rocks on the flanks. An extensive system of valley glaciers still exist. Permafrost is discontinuous in this ecoregion; however, its full extent is unknown. Streams are swift and braided, and most headwaters are in glaciers. Much of the ecoregion consists of rocky slopes, icefields, and glaciers. Where soil development has occurred, principal soils are Lithic Cryorthents, Pergelic Cryaquepts, Pergelic Ruptic-Histic Cryorthents, Typic Cryaquepts, Pergelic Cryumbrepts, and Typic Cryumbrepts. Most soils are stony and shallow over bedrock, or bouldery colluvial, or glacial deposits. Soils on lower slopes and in valleys are typically poorly drained, and have a shallow permafrost table. Most of the region is barren of vegetation. Dwarf scrub communities are most common where vegetation does occur, growing on well-drained, windswept sites. More protected slopes provide moist to mesic sites that support low or tall scrub communities. Open needleleaf forests and woodlands occur on well-drained sites in some valleys and on lower hillslopes.

Boreal Mountains and Plateaus—This ecoregion covers most of northwestern British Columbia and a small area in the extreme southwestern part of the Yukon Territory. The ecoregion is composed of a complex of rugged mountains, high plateaus, and lowlands. Temperature and precipitation vary with altitude. The climate tends to be more moderate in the western half of the ecoregion and becomes more continental towards the eastern part. The typical mean annual temperature for the area is approximately 28 °F with a summer mean of 50 °F and a winter mean of 5 °F. The mean annual precipitation ranges from 16 to 28 in. The vegetation is a complex mosaic, ranging from alpine vegetation and bare bedrock at higher mountain altitudes to alpine fir with some white spruce and deciduous shrubs dominating subalpine forests at middle altitudes in the southern Cassiar and northern Omineca Mountains. Closed canopied forests of lodgepole pine, and white and black spruce dominate the boreal forests of the Stikine and Yukon Plateaus. The most common soils are Humo- Ferric Podzolic soils on upland sites in subalpine regions, and Gray Luvisolic with Dystric Brunisolic soils in the boreal forest regions. Permafrost with low ice content occurs sporadically in the northern part of the ecoregion and is confined to isolated patches in the southwest.

Brooks Range—The Brooks Range ecoregion consists of several groups of rugged deeply dissected mountains carved from uplifted sedimentary rock. Altitudes of the mountain peaks range from 2,600 to 7,900 ft. An arctic climate regime and unstable hillslopes maintain a sparse cover of dwarf scrub vegetation throughout the mountains. The ecoregion is influenced by arctic climate. A weather station located at Anaktuvuk Pass is at an altitude of 2,500 ft. Winter temperatures average a daily minimum of -22 °F and a daily maximum of -8 °F, whereas summer temperatures average a daily minimum of 37 °F and a daily maximum of 61 °F. Mean annual precipitation at Anaktuvuk Pass is 11 in. and annual snowfall is 63 in. Continuous thick permafrost underlies the ecoregion. The principal soils of the Brooks Range are Pergelic Cryaquepts, Pergelic Cryumbrepts, and Lithic Cryorthents. Hillslope soils were formed from local colluvium, whereas most valley soils generally developed from glacial till. Soils throughout this ecoregion typically have poor drainage because of the shallow depth to permafrost. Because of highly erodible hillslope sediments, shallow
soils, high winds, and harsh climate in this ecoregion, vegetation cover is sparse and generally limited to valleys and lower hillslopes. Drier sites support dwarf scrub communities.

Eagle Plains—This ecoregion is almost an entirely unglaciated rolling plateau; it includes the Eagle Plain, Bell Basin, and part of the Porcupine Plateau. The mean annual temperature for the area is 20 °F with a summer mean of 50 °F and a winter mean of -10 °F. Mean annual precipitation ranges from 16 to 18 in. The vegetative cover of this ecoregion is typical subarctic forest. Open, very stunted stands of black spruce and tamarack with secondary stands of white spruce and ground cover of dwarf birch, willow, heath shrubs, cottongrass, lichen, and moss are predominant. On the southern part of the ecoregion, long, even-topped ridges along the Porcupine Plateau have broad, gently rounded summits typical of unglaciated terrain. Relief is low; altitude ranges from 1,000 to 2,000 ft and the highest peak is 3,000 ft. The plain is underlain by Cretaceous and older sandstone and shale. A discontinuous veneer of eolian material covers much of the more stable upper slopes in the region. Permafrost is continuous. High ice content permafrost in the form of ice wedges is common in basin areas. Turbic Cryosols on loamy, inclined, and dissected colluvial material are most common. Regosols on gravelly alluvial material and Dystric Brunisols on sandy colluvium occur on nonpermafrost sites. Characteristic wetlands covering 25 to 50 percent of the land area consist of peat plateau bogs, palsa bogs, and ribbed and horizontal fens.

Interior Bottomlands—This ecoregion is composed of flat to nearly flat bottomlands along large rivers of interior Alaska. The bottomlands are dotted with thaw and oxbow lakes. Soils are poorly drained and shallow, commonly over permafrost. Predominant vegetation communities include forests dominated by spruce and hardwood species, tall scrub thickets, and wetlands. The ecoregion is characterized by a continental climate. The bottomlands in the west receive more annual precipitation than those in the east. Annual precipitation ranges from 11 to 16 in., and annual snowfall ranges from 37 to 80 in. Average daily minimum temperatures in winter range from -27° F to -15 °F. Average daily maximum winter temperatures range from -8 °F to 1 °F. Summer temperatures have lows of about 45 °F and highs of about 72 °F. The terrain of the ecoregion is typified by flat to nearly flat bottomlands, with some inclusions of local hills. Most areas in the bottomlands have a slope gradient of less than 1 degree. Altitudes range from 1,400 ft in the west to 2,000 ft in the east. Fluvial and eolian deposits of mixed origin cover most of the region, but outwash gravel and morainal deposits are in some areas. Meandering streams and side sloughs are prevalent and oxbow lakes and thaw lakes are numerous. Principal soils are Histic Pergelic Cryaquepts, Pergelic Cryaquepts, Aquic Cryochrepts, Typic Cryochrepts, and Typic Cryofluvents. On flat areas away from the main river channels, soils are shallow over permafrost, poorly drained, and nearly always wet. On the slightly higher levees, soils are well drained and permafrost is deep or absent. Soils with permafrost are very susceptible to alteration upon disturbance of the organic mat. Needleleaf, broadleaf, and mixed forest stands occur on a variety of sites in the Interior Bottomlands ecoregion. Tall scrub communities form thickets on flood plains. The wettest sites support a variety of wetland communities, such as low scrub bogs, wet graminoid herbaceous meadows, and wet forb herbaceous marshes and meadows.

Interior Forested Lowlands and Uplands—This ecoregion has a continental climate, with short warm summers, and long very cold winters. Because this ecoregion is so large, temperature and precipitation vary widely from west to east. Total annual rainfall and snowfall generally increase with altitude. Temperature, while affected by altitude, is also influenced by distance from the ocean; maximum summer temperatures increase from west to east, and minimum winter temperatures decrease in the same pattern. Mean annual precipitation over most of the region ranges from 10 to 22 in. with contributions from snowfall ranging from 49 to 81 in. Most precipitation occurs during summer, mainly as a result of convective storms. Average minimum winter temperatures range from 0 °F in the west to -31 °F in the east; average maximum winter temperatures range from 12 °F in the west to -8 °F in the east. Summer temperatures, ranging from 46 to 72 °F with daily fluctuations of 15 to 20 °F, have less regional variation than winter temperatures. The terrain of the ecoregion consists of rolling lowlands, dissected plateaus, and rounded low to high hills. Most of the region lies between altitudes ranging from sea level to 1,600 ft, but some hills rise more than 2,300 ft. Slope gradients are generally from 0 to 5 degrees. The predominant geologic formations are derived from Mesozoic and Paleozoic sedimentary rocks, but extensive areas of volcanic deposits also occur. The region is surficially mantled by undifferentiated alluvium and slope deposits. Streams originating from within this ecoregion tend to be short, whereas larger and longer streams originate from adjacent glaciated mountainous regions. Although thaw lakes and oxbow lakes occur throughout the ecoregion, lakes are not a predominant landscape
feature. The western part of the ecoregion is underlain by thin to moderately thick permafrost, and the eastern part has a discontinuous distribution of permafrost. Dominant soils are Histic Pergelic Cryaquepts, Pergelic Cryaquepts, Aquic Cryochrepts, Pergelic Cryochrepts, Typic Cryochrepts, Typic Cryorthents, and Pergelic Cryumbrepts. The interrelationships among permafrost, surface water, fire, hillslope aspect, and soil characteristics result in a finely textured, complex pattern of vegetation across the ecoregion. Soil temperatures may differ greatly from air temperature, so patterns in vegetation may not correspond with expected site conditions. Needleleaf, broadleaf, and mixed forests occur over a variety of site conditions. Tall shrub communities grow in areas of newly exposed alluvium, such as flood plains, streambanks, drainageways, and lake margins, on burned or otherwise disturbed areas, and near timberline. Low scrub communities occur in moist areas and on north-facing slopes. The wettest sites support tall scrub swamps, low scrub bogs, or scrub-graminoid communities. Recently burned areas display a succession of recovery stages that include mesic forb herbaceous communities, mesic graminoid herbaceous communities, scrub communities, and broadleaf, needleleaf, and mixed forests.

Interior Highlands—This ecoregion is composed of rounded, low mountains, often surmounted by rugged peaks. Although no long-term weather data are available, certain generalizations can be made regarding temperature and precipitation. First, an orographic effect on precipitation causes the highlands to receive more precipitation than the surrounding, lower altitude areas. Second, summer temperatures probably decrease with altitude. Altitudes range from 1,600 ft in the valleys to more than 4,900 ft on the peaks. Slope gradients commonly range from 5 to 15 degrees. The mountains have much more exposed bedrock than the surrounding hills of the Interior Forested Lowlands and Uplands ecoregion. Geologic formations consist of Paleozoic and Precambrian metamorphic rocks, felsic volcanic rocks, and intrusive rocks. The northern part of the ecoregion is underlain by continuous permafrost. Dominant soils are Histic Pergelic Cryaquepts, Typic Cryochrepts, Pergelic Cryumbrepts, Lithic Cryorthents, and Typic Cryorthods. Most soils are shallow, formed in very stony or gravelly material weathered from local rock. The permafrost table is shallow and soils are poorly drained; however, they are generally too shallow over bedrock for ground ice to form. Soils with permafrost are very susceptible to alteration upon disturbance of the organic mat because of the relatively warm (more than 29 °F) permafrost temperature. Organic mat disturbance, such as from wildfires, can result in warmer soil temperatures, lowered permafrost tables, and significant changes in soil physical properties and hydrology. The highest altitudes are barren of vegetation. Dwarf scrub communities, dominated by species of mountain avens, ericads, and willow, are widespread in sites exposed to wind. Lower altitudes are generally more protected from wind and have a denser vegetation cover that can include open needleleaf forests and woodlands. Areas of poor soil drainage support mesic graminoid herbaceous communities.

Mackenzie Mountains—This extremely rugged heterogeneous ecoregion spans the Yukon Territory/Northwest Territories border from Alaska to the Mackenzie Valley. It includes the Ogilvie
and Wernecke Mountains in its westernmost section, the Backbone Ranges in its interior, and the Canyon Ranges to the east. The eastern ranges of the Mackenzie Mountains that lie in the rain shadow of the higher Selwyn Mountains to the west are also included. The ecoregion shows evidence of localized alpine and valley glaciation. The mean annual temperature for the area is approximately 23 °F with a summer mean of 48 °F and a winter mean of -2 °F. Mean annual precipitation is highly variable with the highest amounts, greater than 24 in., occurring in the southwestern part of the ecoregion. Moving westward towards Alaska and the southern Ogilvies, precipitation decreases to approximately 16 in. The region is characterized by alpine tundra at higher altitudes and subalpine open woodland vegetation at lower altitudes. Alpine vegetation consists of lichens, mountain avens, intermediate to dwarf heath shrubs, sedge, and cottongrass in wetter sites. Barren talus slopes are common. Subalpine vegetation consists of discontinuous open stands of stunted white spruce and occasional alpine fir in a matrix of willow and dwarf birch. Permafrost is continuous and of low ice content in most of the Yukon part of the ecoregion. Turbic Cryosols with some Dystric Brunisols and Regosols occur on steeply sloping colluvium.

Ogilvie Mountains—The Ogilvie Mountains ecoregion, located along the eastern edge of the Yukon River Basin, consists of flat-topped hills eroded from a former plain and broad pediment slopes built up from mountains that are much subdued from their former stature. Karst topography is common, and mesic graminoid herbaceous communities and tall scrub communities are widespread throughout the region. The ecoregion has a continental climate. No perennial weather stations are located in this region and thus precipitation and temperature characteristics are interpolated from outside the region. These interpolations indicate that annual precipitation is about 20 in. in the hills to about 26 in. in the higher mountains. Annual snowfall ranges from 51 to 81 in. across the region. Daily winter temperatures range from lows of -26 °F to highs of -8 °F, and daily summer temperatures range from 46 °F to 72 °F. The terrain of the region consists of predominantly flattopped hills eroded from a former plain. Pediment slopes, extending across broad valleys to the foothills of the current, subdued mountains, are characteristics of the plateaus. Erosional scarps in sedimentary rock occur in many localities. Weathered limestone is exposed at higher altitudes, and talus and rubble mantle the lower mountainsides. Altitudes range from 2,900 ft to more than 4,200 ft and slope gradients are generally less than 5 degrees. The region is composed of metamorphic and sedimentary rocks, primarily dolomite, phyllite, argillite, limestone, shale, chert, sandstone, and conglomerate. Karst topography is common and most of the region is underlain by permafrost. Principal soils of the Ogilvie Mountains ecoregion are Histic Pergelic Cryaquepts, Typic Cryochrepts, and Pergelic Cryorthents. Soils were formed in gravelly or stony material weathered from local rock. Soils in valleys were formed from deep, loamy, alluvial sediments from the surrounding uplands. Vegetation is dominated by mesic graminoid herbaceous communities and tussock- forming sedges. Needleleaf, broadleaf, and mixed forest communities occupy lower hillslopes and valleys. Tall scrub communities occur extensively at lower altitudes and can extend above the timberline.

Old Crow Flats—Old Crow Flats is the largest wetland complex in the Yukon Territory. Located on the Old Crow River system north of the Arctic Circle, the Flats contain more than 2,000 ponds and marshes ranging in size from 100 ft2 to 18 square miles. The area is an important breeding and molting ground for 500,000 water birds. Waterfowl, muskrats, and other wildlife of the Flats are of great importance to the Native residents. The ecoregion is unglaciated and incorporates the area of
wetlands and lakes that occupy a glaciolacustrine plain that makes up the lowest part of the Old Crow River Basin. This level, lowrelief ecoregion, locally referred to as “The Flats” lies at about 1,000 ft. The climate is strongly continental. Mean monthly air temperature ranges are as extreme as anywhere in North America. Short warm summers contrast with long very cold winters. The mean annual temperature for the area is approximately 14 °F with a summer mean of 46 °F and a winter mean of -17 °F. Mean annual precipitation ranges from 8 to 10 in. Wetlands, which cover most of the ecoregion, are made up of polygonal peat plateau bogs with basin fens and locally occurring shore fens. Organic Cryosols are the most common wetland soils. Better drained parts of the land support open, very stunted stands of black spruce and tamarack with minor stands of white spruce and ground cover of dwarf birch, willow, cottongrass, lichen, and moss. Static Cryosols on sandy alluvial material and Turbic Cryosols on loamy, ice-rich lacustrine material dominate the mineral soils of the ecoregion. Permafrost is continuous with a high ice content in the form of ice wedges and massive ice bodies.

Pelly Mountains—This ecoregion encompasses the Pelly and northern Cassiar Mountains spanning the British Columbia/ Yukon Territory border. The mean annual temperature for the area is approximately 37 °F with a summer mean of 51 °F and a winter mean of -1 °F. Mean annual precipitation is 20 to 39 in. varying with altitude. Boreal forests of white spruce, black spruce, lodgepole pine, and aspen cover the lower altitude valley bottoms. Much of the ecoregion lies above the treeline and is characterized by alpine tundra communities of lichens, dwarf heath shrubs, birch, and willows. Grasses, sedges, cottongrass, and some mosses occupy wet sites. Open-growing black and white spruce, and alpine fir are prevalent in the subalpine region. The Pelly and Cassiar Mountains, composed of crystalline Mesozoic and Paleozoic strata, are of moderately high relief, ranging from about 4,900 ft to the highest peak at 7,900 ft. Relief is greater in the Pelly Mountains than in the Cassiar Mountains. Permafrost is sporadically distributed. Dystric and Eutric Brunisols are codominant in the ecoregion. Dystric Brunisols are associated with coarse igneous rocks at higher altitudes. Plateau areas with sandy loam morainal parent materials are associated with Eutric Brunisols. Turbic Cryosolic soils are found in alpine areas and in some poorly drained areas.

Ruby Ranges—This ecoregion covers the Kluane River–Kluane Plateau. The climate is characterized by short cool summers and long cold winters. Winter temperature inversions are common, giving milder temperatures at higher altitude. Maritime air from the Gulf of Alaska periodically invades the ecoregion during the winter to produce mild spells with near-thawing temperatures. The mean annual temperature for the area is approximately 27 °F with a summer mean of 50 °F and a winter mean of 2 °F. Mean annual precipitation ranges from 10 to 12 in. Northern boreal forests occupy lower slopes and valley bottoms. Open white and black spruce occur in a matrix of dwarf willow, birch, heath shrubs, and some lodgepole pine. Black spruce, scrub willow, birch, and mosses are found on poorly drained sites. Alpine fir and lodgepole pine occur in higher subalpine sections, whereas the highest altitudes consist of sparsely vegetated alpine
communities of mountain avens, dwarf willow, birch, shrubs and mosses. The terrain consists of rolling to undulating hills above 2,900 ft and the highest peak is 7,560 ft. The most common soils in this ecoregion are Eutric Brunisols on sandy loam morainal or colluvial materials. Regosolic soils are associated with active deposition of gravelly fluvioglacial outwash materials on braided flood plains. Volcanic ash from the 1,300-year-old White River eruption is up to 40 in. thick on lower slopes. In these cases, the soils are classified as either Regosols or Regosolic Turbic Cryosols, depending on the presence or absence of permafrost. Permafrost is extensive and discontinuous over most of the ecoregion decreasing to sporadic along the western side of the ecoregion.

Selwyn Mountains—This ecoregion is located in the Selwyn and southern Mackenzie Mountains that span the Yukon Territory/ Northwest Territories border. For the most part, this is a rugged mountain wilderness, a northern extension of the Rocky Mountains. Climate conditions vary with altitude. The mean annual temperature for major valley systems is approximately 24 °F with a summer mean of 49 °F and a winter mean of -3 °F. Mean annual precipitation is highly variable ranging from 24 in. at lower altitudes on the perimeter of the ecoregion up to 30 in. at high altitudes. The ecoregion is characterized by alpine tundra at higher altitudes and by subalpine open vegetation at lower altitudes. Alpine vegetation consists of crusoe lichens, mountain avens, dwarf willow, and heath shrubs. Sedge and cottongrass are associated with wetter sites. Barren talus slopes are common. Subalpine vegetation consists of discontinuous open stands of stunted white spruce, and occasional alpine fir and lodgepole pine, in a matrix of willow and dwarf birch. Sedge, cottongrass, and mosses occur in wet sites. The Selwyn Mountains, which have been extensively glaciated, are composed of Paleozoic and Precambrian strata intruded by granite stocks. They are divided into several ranges by broad, northwesterly trending valleys. Some ranges contain alpine and valley glaciers. Permafrost is extensive but discontinuous in the western part and continuous with low ice content in the eastern part of the ecoregion. Dystric and Eutric Brunisols on alluvial, fluvioglacial, and morainal veneers and blankets are dominant in the region. Static and Turbic Cryosols with Dystric Brunisols or Regosols are developed on higher altitude, steeply sloping colluvium.

Subarctic Coastal Plains—This ecoregion includes the Yukon River Delta area. Flat, lake-dotted coastal plains and river deltas are characteristics of the region. Streams have very wide and serpentine meanders. Soils are wet and the permafrost table is shallow, providing conditions for wet graminoid herbaceous communities, the predominant vegetation type. Climate in this ecoregion is transitional between maritime and continental influences. In general, the southern part of the has warmer temperatures and receives more precipitation than the northern part. Average annual precipitation is about 20 in. and annual snowfall is about 59 in. Temperatures in winter range from average daily minimums of -13 °F to average daily maximums of 14 °F. Average daily temperatures in summer range from a minimum of 43 °F to a maximum of 55 °F. The terrain of this ecoregion consists primarily of flat poorly drained coastal plains with shallow permafrost tables. Low hills of basalt surmounted by cinder cones and broad shallow volcanic craters occur in some locations, creating a range in regional altitude from sea level to more than 400 ft. Slopes in the plains are generally less than 1 degree. The region is predominantly covered by older coastal deposits of interstratified alluvial and marine sediments. Predominant soils are Histic Pergelic Cryaquepts and Pergelic Cryofibrists. Soils are shallow over permafrost and are constantly wet. Soils have formed from stratified silty and sandy alluvial deposits that, in many areas, have additionally incorporated deposits of volcanic ash and loess. Standing water is almost always present in the ecoregion and wet graminoid herbaceous communities, such as wet meadows and bogs, predominate in saturated soils. Peat mounds, barren sand dunes, and volcanic soils support dwarf scrub communities dominated by ericaceous species.

Wrangell Mountains—The Wrangell Mountains ecoregion consists of steep, rugged mountains of volcanic origin that are extensively covered by ice fields and glaciers. Most slopes are barren of vegetation. Dwarf scrub tundra communities, consisting of mats of low shrubs, grasses, and lichens, predominate where vegetation does occur. Climate is primarily affected by continental influences. Winter low temperatures average -29 °F, and winter highs average 16 °F. Mean summer low temperature is 37 °F, and mean summer high is 72 °F. Average annual precipitation is about 16 in., and annual snowfall is about 69 in. Higher altitudes may receive 80 in. of precipitation annually, including 100 in. of snow. The Wrangell Mountains ecoregion represents a large group of shield and composite volcanoes of Cenozoic age. These volcanic formations lie over Paleozoic and Mesozoic sedimentary and volcanic rocks. The terrain is steep and rugged; most slope gradients exceed 7 degrees and many surpass 15 degrees. Altitudes start at 2,000 ft, most of the largest peaks are 13,000 ft or higher, and several peaks exceed 16,000 ft. Extensive glaciation persists and
permafrost is discontinuous. Much of the landscape consists of steep rocky slopes, icefields, and glaciers. Soil development has resulted in thin, stony soils that are shallow over bedrock or bouldery deposits. Most soils have formed in very stony and gravelly colluvial material. Soils in valleys and on footslopes have formed in glacial till, with a thin mantle of volcanic ash or loess in some places. Principal soils are Lithic Cryorthents, Typic Cryorthents, Pergelic Cryochrepts, and Pergelic Cryumbrepts. Most slopes in the mountains are barren of vegetation. Dwarf scrub communities dominate where vegetation does occur, growing on well-drained, windy sites. Tall scrub communities occur along drainages and on flood plains. Broad ridges, valleys, and hilly moraines at lower altitudes support needleleaf forests dominated by white spruce, or broadleaf forests dominated by paper birch or aspen.

Yukon Flats—The Yukon Flats ecoregion is a relatively flat, marshy basin floor in east-central Alaska that is patterned with braided and meandering streams, numerous thaw and oxbow lakes, and meander scars. In many ways, the ecoregion is similar to the Interior Bottomlands region except that the Yukon Flats ecoregion differs in climatic characteristics. Forests dominated by spruce and hardwood species, tall scrub communities, and wet graminoid herbaceous communities are the predominant vegetation type. The Yukon Flats ecoregion has a continental climate. The mountains surrounding the ecoregion isolate it from the weather systems affecting the neighboring regions. Consequently, summer temperatures tend to be higher than at other places of comparable latitude and winter temperatures tend to be colder. Average daily temperatures in winter range from lows of about -29 °F to highs of about -11 °F. Average daily temperatures in summer range from lows of just about freezing to highs of about 72 °F. Annual precipitation is low, averaging 6.5 in. and average snowfall is 45 in. (water content of about 4 in.). Local precipitation is not sufficient to maintain water levels in many lakes. Levels are primarily maintained by the yearly flooding of the region by the Yukon River that accompanies spring breakup of ice (Gallant and others, 1995). The central part of the ecoregion is flat, whereas the edges of the region range from 300 ft to more than 600 ft. Slope gradient is generally less than 1 degree in the center and 1 to 2 degrees at the edges. The region is mantled by Quaternary-age alluvial deposits. The Yukon River drains the ecoregion, assisted by numerous meandering and braided tributaries and side sloughs. Permafrost is present in most areas, except beneath rivers and large thaw lakes. Thaw lakes and oxbow lakes are abundant.
Principal soils are Histic Pergelic Cryaquepts, Pergelic Cryaquepts, Aquic Cryochrepts, and Pergelic Cryochrepts. Most soils were formed from silty alluvium and loess from the flood plains of the Yukon River. On flat areas away from the main river channels, soils are poorly drained, are commonly overlain by peat, and have a shallow permafrost table. Soils on natural levees are better drained and consist of silty and sandy sediments. Needleleaf, broadleaf, and mixed forests are widespread and occupy sites representing an array of soil drainage characteristics. Tall scrub thickets occur on alluvial deposits subject to periodic flooding. Tall scrub swamps and wet graminoid herbaceous communities occupy the wettest sites.

Yukon Plateau Central—This ecoregion extends northward from Lake Laberge to the lower Stewart River in the central Yukon. The Yukon Plateau Central ecoregion is composed of several groups of rolling hills and plateaus separated by deeply cut, broad valleys. The climate is cold and semiarid. The mean annual temperature for the area is approximately 26 °F with a summer mean of 54 °F and a winter mean of -2 °F. Mean annual precipitation ranges from 10 in. in the southern areas near Carmacks to 16 in. at higher altitudes in the north and east. White and black spruce form the most common forest types. Black spruce is usually dominant in wetter areas. Lodgepole pine commonly invades burnt-over areas and very dry sites. In some places, alpine fir forms the treeline but is sparse and is usually associated with white spruce and occasionally with paper birch. Sedge tussocks and sphagnum are common in wetlands. Scrub birch and willow occur in subalpine sections that extend up to the treeline. A significant vegetative feature of this ecoregion is the presence of extensive grasslands on all low-altitude, south-facing slopes. The forests suffer frequently from recurring natural fires such that series of ecological communities are most common. Altitudes are above 3,300 ft, except for major river valleys, which lie below 2,000 ft in the northwestern part. Several mountains reach heights of 4,900 ft. Eutric Brunisols, which developed on steeply sloping, ridged-to-hummocky, loamy morainal and sandy fluvioglacial material, are dominant in the ecoregion. Much of the ecoregion is covered by a veneer of recent volcanic ash 4-12 in. thick. Permafrost is discontinuous to sporadic with high ice content associated with fine-textured valley deposits. Turbic Cryosols are confined to wet depressions and beneath mature forests on lower, north-facing slopes.

Yukon Plateau North—This ecoregion lies within the Stewart, Macmillan, and Pelly Plateaus and the southern foothills of the Selwyn Mountains. The terrain includes rolling uplands, small mountain groups, and nearly level tablelands dissected by deeply cut, generally broad, U-shaped valleys. The Tintina Trench, a straight, steep-sided valley 3-12 miles wide, traverses the ecoregions from southeast to northwest. The mean annual temperature for the area is approximately 25 °F with a summer mean of 51 °F and a winter mean of -4 °F. Mean annual precipitation ranges from 12 in. in the major valleys up to 24 in. in the mountains to the northeast. Northern boreal forests exist at altitudes up to 4,900 ft. White spruce in a matrix of dwarf willow, birch, heath shrubs, and occasionally lodgepole pine, form extensive open forests, particularly in the northwestern part of the ecoregion. Black spruce, scrub willow, birch, and mosses are found on poorly drained sites. Alpine fir and lodgepole pine occur in higher subalpine sections. Extensive discontinuous permafrost with a medium ice content is widespread, decreasing to sporadic discontinuous permafrost along the southwestern edge of the region. Turbic Cryosolic and Eutric Brunisolic soils predominate, and occasional pockets of Dystric Brunisols occur on coarse-textured morainal and fluvioglacial materials.

Yukon Southern Lakes—This ecoregion extends from Lake Laberge south to the boundary with British Columbia. The climate is cold and semiarid. In major valleys, the mean annual temperature is about 28 °F with a summer mean of 50 °F and a winter mean of 2 °F. Lying within the rain shadow of the St. Elias Mountains, mean annual precipitation ranges from 9 to 12 in. in the major valleys. Boreal forests are composed of open white spruce and lodgepole pine intermixed with aspen. South-facing slopes at low altitude are occupied by grassland communities. Subalpine altitudes above 4,000 ft support open forest communities of alpine fir, white spruce, and some lodgepole pine. Most of the terrain lies 2,000 to 4,900 ft in altitude, but a few peaks are higher than 5,900 ft. Underlain by Mesozoic sedimentary strata and Paleozoic metamorphic slates and schists, the topography is characterized by dissected plateaus and rolling hills. Eutric Brunisolic soils on sandy loam and rolling morainal to steep colluvial material are dominant. Low ice content permafrost occurs in a sporadic discontinuous pattern. Cryosolic soils are scattered throughout the landscape on some poorly drained areas and on north-facing slopes.

Yukon Stikine Highlands—This ecoregion covers a zone of climate transition from coastal to interior conditions in northwestern British Columbia and southern Yukon. The ecoregion falls within the rain shadow of the Coast Mountains. Precipitation decreases moving inland, and temperatures are moderated throughout the year by the influence of maritime air masses. The mean annual temperature for the area is approximately 30 °F with a summer mean of 50 °F and a winter mean of 8 °F. Mean annual precipitation ranges from 20 to 24 in. The ecoregion is composed of a combination of three distinct vegetation zones: alpine tundra dominated by low-growing heather, dwarf birch, willow, grass, and lichen; subalpine forests of alpine fir, white spruce, and an occasional Engelmann spruce; and closed boreal forests of black and white spruce. Permafrost is discontinuous and sporadic with generally low ice content. Soils range from Brunisolic and Regosolic with some Cryosolic soils in alpine regions to Dystric and Eutric Brunisols in subalpine and boreal sections of the ecoregion.

Hydrologic Characterisitics of the Yukon River Basin

Surface Water, Sediment, Water Quality

Surface Water

Streamflow quantity and variability have considerable influence on the quality of surface water. The quantity of water in a stream or river influences its ability to support aquatic communities, to assimilate or dilute waste discharges, and to carry suspended sediment and geochemical weathering products. Temporal variability of streamflow may, in turn, cause temporal variability of water quality. Thus, knowledge of streamflow is important to understand the water-quality and ecological dynamics of a watershed. The Yukon River is composed of many streams and rivers. Utilizing the Alaska Hydrologic Unit Classification system (U.S. Geological Survey, 1987) and a somewhat similar classification system for Canada, the Yukon River Basin can be divided into 13 major basins (table 5; fig. 17). These basins represent the eight major tributaries to the Yukon River and the major lowland areas that drain directly into the Yukon River.

Streamflow

Generally, most stream-gaging stations in the Yukon River Basin are located on rivers that drain areas larger than 1,000 square miles. Sixty-eight stream-gaging stations operated by the USGS in Alaska and the Water Survey of Canada in the Yukon Territory and British Columbia have 10 or more years of record (fig. 18; table 6). Stream-gaging stations have been located on most of the main tributaries to the Yukon River. In 1999, 26 stream-gaging stations were active in the Yukon River Basin: 17 in Canada and 9 in Alaska (fig. 18). Active stream-gaging stations were located on seven of the 13 major basins (4 in Canada, 3 in Alaska).

Water Quality

The water quality of the Yukon River Basin is important for many reasons. Residents who live along the main stem of the Yukon or its tributaries use the surface water for drinking. Salmon and other fish species require adequate water quality for their survival as does the abundant wildlife present in the basin.
Similar to the suspended-sediment data, records of all waterquality measurements are available in USGS publications (U.S. Geological Survey 1954-62, 1971, 1976, 1972-75, 1976-97). Data are also stored in electronic format in the USGS National Water Information System (NWIS). Water-quality data from the Canadian Yukon are available in electronic format from the world wide web (http://www.ec.gc.ca/water/index.htm).