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 Copper River Delta coastal plain 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.