One-Third of Alaska Water Finds Its Way to the Yukon

While driving back from a recent trip to Circle, a friend frowned at the suggestion that the Yukon River, the near-frozen waterway into which we had just tossed chunks of ice, was not the longest river in the United States. He wasn’t satisfied until his wife held up a map with Alaska superimposed over the Lower 48. With some dejection, he noted that the Mississippi was indeed longer than the Yukon, as is the Missouri.

Alaska’s longest river is no slouch (or slough) though, carrying water almost 2,000 miles from its birthing place high in the Yukon Territory, across Alaska from east to west, then finally to the Bering Sea. Hydrologist Timothy Brabets and a few of his colleagues at the U.S. Geological Survey in Anchorage took a good look at the Yukon River Basin in a paper published in 2000. I siphoned the following facts from his paper:

•Counting its headwater basins in Canada, the Yukon River drains 330,000 square miles, an area the size of Turkey. In Alaska proper, one-third of all flowing water finds its way into the Yukon. If not evaporated, locked in ice, or absorbed, water reaches the Yukon from places as far flung as Arctic Village, Cantwell, Nulato, and Northway.

• Two rivers—the White River that drains the Wrangell-St. Elias Mountains and the Tanana River that drains the north side of the Alaska Range—account for 29 percent of the Yukon’s total water flow.

• The White and Tanana rivers are largely responsible for making the Yukon brown and silty over much of its length. They carry powdered rock from the Wrangells and the Alaska Range and dump it by the ton into the Yukon.

• The Yukon transports 60 million tons of silt, sand, and clay each year to the Bering Sea.

• The Yukon deposits 20 million tons of silt, sand and clay each year onto its flood plains and gravel bars.

• The Yukon’s two major tributaries to the north are the Porcupine and Koyukuk rivers. Together, they drain an area of 80,000 square miles, more than the combined inputs of the White and Tanana rivers.

• Despite draining a larger area than that drained by the White and Tanana rivers, the Porcupine and Koyukuk feed much less water to the Yukon. The absence of glaciers in these two basins deprives them of ice melt during the summer months, and permafrost underlying the northern river valleys doesn’t allow for much groundwater flow.

•During the winter, the Porcupine and Koyukuk rivers sometimes cease to flow beneath the ice.

• The Nenana River, which carries meltwater from glaciers, and the Chena River, which does not, are almost equal in basin size, yet the discharge from the Nenana is 2.5 times that of the Chena, and the Nenana carries 30 times the sediment load.

• Glaciers cover just one percent, about 3,500 square miles, of the Yukon River basin.

• The Yukon moves more than 95 percent of all its sediment in May through September. During October through April, the river is almost clear.

• At least one quarter of all the powdered mountain carried to the Yukon by glacial streams ends up on riverbanks or gravel bars. A large proportion of that sediment does not reach the sea for a century.