The Delta River (Middle Tanana: ) is a river in eastern Alaska, United States. It is a tributary of the Tanana River, and by extension the Yukon. Originating from the Tangle Lakes, the river flows north through the Alaska Range, carving a valley perpendicular to the east-west mountain range. Initially clear and slow-moving, it becomes faster and silty after taking in Eureka Creek, its first major tributary. As it flows through the Alaska Range, it takes in meltwater from various glaciers and glacier-fed streams. At the end of its roughly course, the river meets the Tanana near the settlements of Big Delta and Delta Junction. The geology of the basin is highly complex, with various glacier-shaped landforms and alluvial deposits from the river. Dust from the river valley is spread by the wind and forms a layer of loess to the west of the river. The river basin is home to various birds, mammals (including caribou), and plants, while the water hosts fish such as northern pike and arctic grayling.
The uppermost portions of the Delta River basin are in the Tangle Lakes Archaeological District, containing over 600 documented archaeological sites, with the earliest occupation appearing to be from over 10,000 years ago. The Upper Tanana Athabaskans and the Ahtna both lived in the basin and hunted around it. In 1885, American army officer Henry Tureman Allen explored the river and named it for its braided water channels. Gold mining and road construction took place in the river valley during the early 1900s, followed by the construction of the Richardson Highway running parallel to the river. Agricultural development projects were made around the river during the 1970s and 1980s, although most of the river basin remains wilderness under the administration of the Bureau of Land Management and the Alaska Department of Natural Resources. A portion of the upper Delta River was designated a National Wild and Scenic River in 1985.
Course
The Delta River flows for about through central Alaska, flowing north through a pass in the Alaska Range to the Tanana River. The source of the river is Lower Tangle Lake, the lowest of the Tangle Lakes chain to the west of Paxson, Alaska.
alt=A fisherman stands by the shores of a large river amidst a rural grassy environment with huge snowy mountains |thumb|A fisherman by the Delta River in its pass through the Alaska Range
alt=A river runs through a small rocky waterfall in a mountain range|thumb|A waterfall in the upper Delta River
From its source, the Delta flows at a low velocity for about north, through a series of small pools and riffles, before descending a waterfall. After this fall, the river narrows from in width and gains an increased velocity. About upstream from the falls, the river slows and widens again, splitting into several channels with sand and gravel bottoms. About another mile further upstream, it slows more, and widens to a channel about wide and deep.
About from its source, the Delta River receives its first major tributary along its western bank, Eureka Creek, which carries meltwater from various glaciers along the southern face of the Alaska Range. This is followed by various other minor tributary streams until its confluence with Phelan Creek upriver, which drains the Gulkana Glacier. After this, it enters a narrow pass running through the Alaska Range, and flows north through the mountains for about , as it receives meltwater from glaciers and various steep mountain streams. Among the largest glaciers in the region, the Black Rapids Glacier, drains into the river at a site called Black Rapids. The average elevation of the river during this section is about , flanked on either side by the adjacent mountains of the Alaska Range, which range in elevation from at their summits.
About after Black Rapids, the valley grows shallow, and the Delta runs through an expanse of glacial moraines for about . After this, it flows through a wide stretch of glaciofluvial fans for the last of its course. At the beginning of this section, it lies in a valley that descends to up to beneath the surface of the fans, decreasing to about near its mouth. About from its mouth, the Delta takes in its last major tributary, the long Jarvis Creek, which originates from Jarvis Glacier on the north face of the Alaska Range. The Delta River meets the Tanana River adjacent to the settlement of Big Delta, Alaska, which eventually drains into the Yukon River and thus the Bering Sea.
Hydrology
thumb|The Delta River is named for its heavily braided channels.The water of the Delta River is extremely clear from its source to its confluence with Eureka Creek, after which it becomes more silty and fast-moving. Its broad flood plain is interrupted by various braided channels. It presents little flooding risk to communities along the river, as its western bank (on which no settlements are located) is lower than its east bank. However, lateral erosion along the eastern bank does pose a threat to homes and roads, prompting the installation of groynes (flow-regulating structures) along the bank by the early 1980s.
Large glaciers in the Alaska Range such as the Black Rapids Glacier, Canwell Glacier, and Castner Glacier run through the mountain valleys and drain into the Delta, as do various glacially-fed mountain streams.
Basin environment
The Delta's drainage basin spans around of territory in central Alaska. It covers portions of four of Alaska's physiographic provinces as defined by Clyde Wahrhaftig's 1965 survey. It begins in the Gulkana Upland, a series of high ridges separated by wide, hummocky (mound-covered) valleys; at the southernmost portion of the region are the Amphitheater Mountains, which contain a few small cirque glaciers and snow fields, as well as small mountain streams which feed the Delta in the early portions of its course. To the north, the basin runs through the Alaska Range itself, a rugged mountain range drained by glaciers on its northern and southern faces. A stretch of the Northern Foothills of the Alaska Range also lies within the drainage basin, comprising a series of flat-topped ridges running in parallel to the mountains. The lowest portion of the drainage basin is within the outwash plains of glaciofluvial deposits along the southern floodplains of the Tanana River, defined as part of the Tanana-Kuskokwim Lowland.
The basin is in a subarctic climate, where temperatures can reach below during the winter. Daily high temperatures during the summer can occasionally exceed , although freezing temperatures have been recorded in all months. Freezing temperatures are generally reached in October, with the spring thaw emerging around late May or early June. Summers are short, dry, and relatively warm, while winters are long and cold.
Groundwater
Alluvial fans near the Delta and Gerstle River create a large aquifer in the surrounding region. Although the lower portions of the Delta River receive some groundwater discharge, both the Delta and Jarvis Creek ultimately lose water to it, serving as a source of groundwater recharge. Snowmelt during the summer results in the water table rising more than , which falls again during the winter as meltwater streams dry up.
Within the surrounding region, discontinuous permafrost can be found. In some areas, it is found directly below the surface, while the permafrost layer can begin as deep as in others. Permafrost is absent from the area directly adjacent to and beneath the Delta River itself. Shallow permafrost causes poor drainage and wet soils on a local level, although its discontinuous nature prevents large-scale impediments to groundwater recharge.
Geology
alt=A black and white photograph of the lower course of the Delta River, showing various braided streams and a large cloud of silt blowing into the air|thumb|The lower Delta as seen from the air, showing clouds of silt carried by a strong east wind, 1948The geology of the Delta River area is highly complex, including both Precambrian-era metamorphic rocks and recent volcanic and alluvial deposits. Much of the region is in the Wrangellia Composite Terrane, a crust fragment containing primarily oceanic rocks, with basalts, carbonate rocks, and mafic and ultramafic rocks as intrusives. Various glacial deposits from the Quaternary period are found throughout the area, forming eskers, moraines, kettles, kames, and outwash plains.
Besides the Delta, three other rivers cut a north-south track through the Alaska Range: the Nenana, Nabesna, and the Chisana. These likely predated the uplift of the Alaska Range around 25 million years ago, eroding a valley along their original course as the range rose.
The upper portions of the basin, between the Alaska Range and the Amphitheater Mountains, contain large amounts of economically-useful minerals, such as platinum group metals, nickel, cobalt, and copper. Small-scale mining operations have occasionally mined gold in the region.
Soils and sediment
The soils of the Delta River basin consist of brown forest soil, permafrosted gleysol (grayish waterlogged soil), alpine podzol, and lithosols (shallow, rocky soils). The brown forest soil is mainly found at low elevations, while the lithosols and alpine podzols are found at high elevation in the mountains. Gleysol is generally found in depressions in the central and southern parts of the basin. Organic deposits such as peat are found across the river basin, especially in depressions along the flood plains and terraces. Peat deposits in nearby bogs and fens can reach several feet in thickness.
Much of the region in the Delta River basin north of the Alaska Range is covered in a layer of loess (sediment consisting of wind-blown dust), originating in the floodplains of the Delta River and Jarvis Creek. This layer ranges from thick in the area west of the river, but is generally less than thick in the area east of it. The loess began to be rapidly deposited after the end of the Last Glacial Period around 12,000 years ago, as the receding glaciers exposed the ground. Loess accumulation from flood plain sediment continues, with the thickest deposits on hills in the northern basin.
Biology
alt=Two caribou run across a stretch of whitewater rapids towards a rocky cliff|thumb|Two [[Reindeer|caribou running across the river]]
About 33 species of mammals are attested in the Delta River basin. These include caribou of the Nelchina herd, Dall sheep, and moose. Black bears are frequently found on the flood plains and stream terraces, while grizzly bears are mainly found in the uplands, descending into the river valley during salmon spawning season. Native hunting trails and routes ran through the Delta River valley, often passing over the tops of eskers and the rims of plateaus.
A band of the Upper Tanana Athabaskans lived on the north slopes of the Alaska Range along the lower Delta and Goodpaster Rivers, hunting caribou, dall sheep, and moose. The Ahtna, present across the Copper River basin to the south of the Alaska Range, lived in the portions of the Delta watershed above the Black Rapids. The Delta valley may have been the main troute between the Upper Tanana and the Ahtna, who had overlapping territories in the region, as well as some trade and marriage relations. The photographer and adventurer James T. Geoghegan in 1904 recorded "Thothendig" as a native name for the river. The Middle Tanana name for the river is , meaning "sandy gravel bed river", a name shared by the Johnson River and Delta Creek.
The American army officer Henry Tureman Allen named the Delta River during his 1885 expedition, naming it after its braided channels, which he found reminiscent of the networks of distributaries found in river deltas. An 1898 expedition by another army officer, Edwin Forbes Glenn, became the first westerners to travel through the Delta valley following advice from indigenous guides, who judged it to be the easiest route through the Alaska Range. Gold was discovered near Eureka Creek around 1900, prompting the creation of the Eureka Creek Mining District and around 250 miners to work in the area over the following decade.