THE NATURAL ENVIRONMENT
The Yukon Region comprises the lowlands and plains of central and western Alaska and the interior highlands drained by the Yukon River and its tributaries. It extends from the Bering Strait to the Canadian border and from the Brooks Range in the north to the Alaska Range in the south. The area is approximately 130,000,000 acres, or 35 percent of the state. About 20 percent of the state's population resides here in communities ranging in size from small villages in the lower Yukon to the Fairbanks metropolitan area.
Because of its diversity and expanse, the Joint Federal-State Land Use Planning Commission divided the region into six subregions to facilitate data collection and description of various topics (Figures 1 and 2). The subregional approach is used for some topics in this profile, but most information is presented regionally or grouped by physiographic features and landforms.
Lower Yukon Subregion—This subregion includes the coastal area north of the Kuskokwim River to Norton Sound, the lowlands along the Yukon River to Kaltag, and part of the Nulato Hills and Kuskokwim Mountains. The area is drained by the Yukon River and its tributaries and by streams flowing into the Bering Sea and Norton Sound. It encompasses 37,900 square miles (98,161 km2).
The Yukon Delta consists of lowlands and tidal flats containing thousands of lakes, ponds, and tundra hummocks. There are a few upland areas of significance such as the Asikinuk Mountains and bluffs at Cape Romanzof. North and west of the Yukon River highlands are the southern regions of the Nulato Hills and the principal river systems—the Andreafsky, Bonasila, Anvik, and Kaltag.
On the south and east side of the Yukon the Innoko and its major tributary streams, the Iditarod, Dishna, North Fork, Mud, and Yetna, are primary. Much of the terrain is low elevation with many lakes and ponds through which rivers snake their way, leaving oxbow lakes and isolated bends. Stream channels change from time to time because of frequent spring floods. The north slope of the Kuskokwim Mountains, which forms the south boundary of the unit, contains ridges and peaks ranging from 1,000 to 3,000 feet (304.8 to 914.4 m). The Beaver Mountains reach 4,150 feet (1,264.9 m), while the subregion's highest peak, Cloudy Mountain, attains a height of 4,234 feet (1,290.5 m).
Offshore are Pinnacle, Hall, and St. Matthew Islands totaling about 80,000 acres (32,376 ha). The islands are located in the Bering Sea some 220 miles (352 km) west of Hooper Bay. They are characterized by a series of volcanic ridges with low valleys between. Highest elevations are the volcanic cones of Sugarloaf Mountain at 1,380 feet (420.6 m), and two unnamed peaks at Cape Upright, 1,505 feet (458.7 m), and near Glory of Russia Cape, 1,475 feet (449.6 m) above the sea. There are a few lagoons on St. Matthew and a few freshwater lakes. Hall Island rises abruptly from the sea. It is located northwest of St. Matthew and contains a single mountain mass that reaches 1,665 feet (507.5 m). All but the southeast shoreline of Hall Island is very steep. Pinnacle Island contains a scenic spire and needle formations that reach 1,250 feet (381 m) above the sea. There are extreme winds in this area, and summer fog is common. All three islands are uninhabited and constitute the Bering Sea National Wildlife Refuge.
Central Yukon Subregion—The Central Yukon Subregion includes the Yukon River and its drainages from Kaltag upstream to Rampart and contains approximately 19,000 square miles (49,210 km2) of land and water. The north boundary is generally the divide north of the Melozitna and Nulato Rivers. The south boundary follows the divide in the Kaiyuh Mountains from Kaltag to Poorman and then the drainage divide on the Kuskokwim Mountains to Tanana and Rampart.
About 20 percent of the area within the basin of the Yukon-Tanana River consists of swampy lowlands underlain by alluvial, colluvial, and eolian unconsolidated deposits, most of which are perennially frozen. The lowlands are bordered by the Kaiyuh Hills, Kokrines Hills, Ray Mountains, and Kuskokwim Mountains—a succession of northeast-trending ridges with rounded to flat summits rising from 1,000 to 4,000 feet (304.8 to 1,219.2 m) topped by isolated groups of rugged glaciated mountains 3,000 to 5,500 feet (914.4 to 1,676.4 m) in altitude. Valleys have flat floors one to five miles (1.6 to 8 km) wide.
The principal river and river systems of the subregion are the Nulato, Kaiyuh Slough, Yuki, Nowitna, Melozitna, and Tozitna. Other water features include thousands of lakes, ponds, and oxbow lakes, many of which are located in the Kaiyuh Slough, lower Nowitna River basin and upper Melozitna and Little Melozitna River areas, and along the Yukon valley. The Melozi, Little Melozitna, and Horner Hot Springs are also in the subregion.
Koyukuk Subregion—The Koyukuk Subregion is drained by the Koyukuk River and its tributaries from the divide in the Kokrines-Hodzana highlands-Philip Smith Mountains west of the Nulato Hills and contains about 33,000 square miles (85,470 km2) of land and water. The Koyukuk River heads in the Endicott Mountains in the central Brooks Range and flows mostly southwest some 320 miles (512 km) to its confluence with the Yukon near the village of Koyukuk. The north boundary is the continental divide of the central Brooks Range, extending from the Helpmejack Hills to the headwaters of the Alatna River on the west and to the head of the Dietrich River on the east. The Endicott Mountains, east-trending and separated by deep, glaciated valleys, cover the northern part of the subregion. The peaks of the Endicotts range from 5,000 feet (1,524 m) to the highest, Mt. Doonerak, at 7,610 feet (2,319.5 m).
Southward the Brooks Range descends through low mountains and uplands to the marshlands of the Koyukuk River. South of the Koyukuk River the subregion consists of rolling uplands and several isolated groups of rugged mountains. The south boundary is the divide north of the Melozitna River in the Ray Mountains. The west boundary follows the continental divide across Purcell Mountain, the Zane Hills, and Lockwood Hills and on to the Helpmejack Hills.
The major tributary rivers and river systems of the Koyukuk Subregion are the Alatna, Wild, John, North Fork, Koyukuk-Tinayguk, Middle Fork, Koyukuk-Dietrich, South Fork, Gisasa, Honhosa, Kateel, Huslia, Dagatli, Indian, Little Indian, Hogatza, and Dulbi.
Upper Yukon Subregion—The Upper Yukon Subregion occupies about 60,000 square miles (155,400 km2) in the northcentral part of the state. It covers all of the drainage into the Yukon River in Alaska from the Tanana River upstream. The northern boundary is the continental divide on the crest of the Brooks Range. It is bounded on the east by the U.S.-Canada border and on the south by the Tanana-Yukon divide. The subregion is drained by the upper Yukon River and its tributaries from the Canadian boundary west to Rampart. It includes the Yukon Flats, the Porcupine plateau, the margins of the Kokrines-Hodzana highlands, and the Yukon-Tanana upland.
The Yukon Flats consist of marshy, lake-dotted flatland rising from 300 feet (91.4 m) in altitude on the west to 600 to 900 feet (182.9 to 274.3 m) on the north and east. The boundaries with the surrounding uplands are gradational. The flats are underlain by fine lake sediments and are thought to be the site of a late Tertiary lake. The Porcupine plateau consists of low ridges with gentle slopes and rounded to flat summits 1,500 to 2,500 feet (457.2 to 762 m) high. A few domes and mountains rise as high as 5,800 feet (1,767.8 m). The Yukon-Tanana uplands are northeast-to east-trending, rounded ridges rising to 1,500 to 3,000 feet (457.2 to 914.4 m). Valleys are generally flat.
Major drainages within the subregion include Birch, Beaver, and Hess Creeks, the entire drainage of the North, Middle, and East Forks of the Chandalar River; the Hadweenzic, Hodzana, and Ray Rivers; the Porcupine River and its tributaries; the Coleen, Sheenjek, Koness, and Christian Rivers; part of the Old Crow River system; the Black, Little Black, and Salmon Trout Rivers within Alaska; Seventymile and Charley Rivers; and the lower parts of the Tatonduk, Nation, and Kandik Rivers.
The subregion contains the north flank of the central Alaska Range from near Mt. Foraker in Mt. McKinley National Park to the Canadian border. The central and eastern parts of the Alaska Range consist of two or three parallel rugged glaciated ridges 6,000 to 9,000 feet (1,828.8 to 2,743.2 m) high topped by extremely rugged snow-capped mountains more than 9,500 feet (2,895.6 m) in altitude. Mt. McKinley, 20,320 feet (6,193.5 m) high, is in this part of the range. The almost continuous wall of the Alaska Range is breached by the Nenana, Delta, Nabesna, and Chisana Rivers and Mentasta Pass which provide access through the range. The subregion is characterized by braided glacial streams heading in the Alaska Range.
Principal drainages include the Tanana River; the Chisana, Nabesna, and Tetlin river systems; the Tok-Little Tok river system; the Robertson, Johnson, and Delta Rivers; the Goodpaster and Healy Rivers; the Salcha, Chena, Chatanika, and Tolovana-Tatalina Rivers; the Nenana-Teklanika, Totatlanika, Little Delta, and Wood Rivers; Tatlanika, Clear, and Delta Creek drainages; and the Kantishna-Toklat-McKinley, Zitziana, Cosna, and Chitanana Rivers.
Upper Yukon-Canada—The Upper Yukon-Canada Subregion includes the parts of the White, Fortymile, and Ladue river drainages within Alaska and covers about 9,000 square miles (23,310 km2). There are two distinctly separate parts of the subregion.
The north part of the subregion is drained by the Fortymile River and its tributaries. The south part is the Alaskan part of the area drained by the upper White River and its tributaries. The north part includes the eastern part of the Yukon-Tanana upland comprised of rounded, even-topped ridges 3,000 to 5,000 feet (914.4 to 1,524 m) in altitude with some domes rising as high as 6,800 feet (2,072.6 m) that are separated by broad alluvium-floored basins. The south part includes an area from the summit and north slopes of the Wrangell Mountains north to the Nutzotin Mountains.
Major drainages include the White River; Snag, Beaver, and Ptarmigan Creeks; Fortymile and Ladue Rivers; and the North, Middle, Mosquito, West, and Dennison forks of the Fortymile.
Tanana Subregion—The Tanana Subregion is entirely within the drainage basin of the Tanana River and covers about 45,000 square miles (116,500 km2) of area. It is an area of diverse physiography which includes the southern part of the Yukon-Tanana upland, the Tanana-Kuskokwim lowland, the eastern part of the Kuskokwim Mountains, the central and eastern part of the Alaska Range, and a part of the Wrangell Mountains.
The Yukon-Tanana upland consists of rounded hills around a high central area of rugged mountains. In the western part the northeast-trending ridges rise to 1,500 to 3,000 feet (457.2 to 914.4 m) surmounted by groups of peaks 4,000 to 5,000 feet (1,219.2 to 1,524 m) high. In the eastern part the ridges are 3,000 to 5,000 feet (914.4 to 1,524 m) high with no particular orientation. Some have domes as high as 6,800 feet (2,076.6 m).
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