The Mighty Copper River

Rolling down the river mighty happy. The Copper River and its tributaries make up an impressive watershed of wilderness, approximately 26,500 square miles (nearly 16 million acres) encompassing the far corners of four distinct mountain ranges including the Alaska Range, the Wrangell-St. Elias Mountains, most of the Chugach Mountains and a small section of the Talkeetna range. They contain sources of both volcanic and sedimentary rock from some of North America’s tallest peaks, as well the largest subpolar icefield in the world, the Bagley Icefield, making the Copper River Watershed one of the most prolific and diverse glacial deposits in the world.

On May 28, 2013, a NASA satellite captured this image of the Copper River Delta showing extradorinary sediment load in the Gulf of Alaska. The Childs and Miles glaciers, as well as the Bagley Icefield drain into the Copper River. As the glaciers slide down the mountains into the valleys, the they grind on the bedrock below, creating what scientists refer to as “glacial flour.” That sediment is a good source of iron and nutrients for phytoplankton and marine plants, which in turn support abundant salmon runs.

Mountainous glaciers blanketing the basin have been melting since the last ice age ended 12,000 years ago depositing massive amounts of glacial silt downstream, upwards of 60-100 million tons per year! The lower watershed, the Copper River Delta, is a coastal plain that spans over 700,000 acres and stretches almost seventy-five miles along the coast.

The Copper River drains glacial silt from along its entire path. During summer months, the daily sediment load can be 750,000 cubic feet of mud and sand; much of the river is frozen in winter from late November to April. The Copper River carries one of the largest river sediment loads known and over the past eight thousand years, the river has built up a layer of silt over 600 feet deep as it leaves the Chugach Mountains and enters the coastal plain. These depositions have formed the Copper River Delta.