The Fortymile is a swift flowing river with an average grade of 10 feet per mile. While most of the dralnage is located in Alaska, the last 23 miles of the river flows through the Yukon, emptying into the Yukon River 46 miles downstream from Dawson City.

The placer reserves of the Canadian section of the Fortymile are divided into two categories: dredging reserves which are found in the wetted perimeter of the river valley, and bench deposits which are found above and adjacent to the river channel. Depth to bedrock averages 12 to 15 feet. In the bench reserves, bedrock is above the water table. The gravel is usually frozen but some pockets are thawed. The gravel is overlain with muck, usually frozen, with a depth of from 6 inches to 30 feet. Gravel depth is from 6 feet to 30 feet with 20 feet being average. While the Fortymile became famous as a coarse gold area, the Canadian section of the river appears to have extremely fine gold throughout the entire gravel section.

Gold was first discovered on the Fortymile River in 1886, precipitating the first major Yukon gold rush. In 1887 more than 14,000 ounces were mined with pick, shovel, and rocker, by some 200 miners. The town of Fortymile was established at the confluence of the Fortymile and the Yukon Rivers. As well as fine bar gold, coarse nuggets were being found. The Fortymile district was the first area in which wood fires were used to thaw shafts in order to gain access to the rich gravel and coarse gold at bedrock depth.

Between 1906 and 1911, a dredge worked the Fortymile 8 miles upriver from the mouth, recovering over 8,000 ounces of gold. This project was abandoned with the advent of the First World War. In the early 1930’s, another dredging operation was initiated 11 miles upriver from the mouth, at the confluence of Bruin Creek and the Fortymile River. On the American section of the river, mining has been continuous and extensive with numerous dredging and cat operations.

More than 800,000 ounces of gold have been recovered from the Fortymile.