A Guide to the Alaska Highway ("Alcan")

This interpretive sign at Km 668.2 of the Alaska Highway explains the flash floods that are common in the area, and the type of gravel deposit known as an alluvial fan seen across the river.

The Northern Canadian Rockies are famous for their summer downpours. When heavy rains fall on mountains largely bare of trees and other soil-holding vegetation, the water carries sand, gravel and even boulders into the gullies between the peaks. Everything washes out onto the flat valley floors.
This is one such deposit. It is called an alluvial fan because its outline resembles an open fan. Material carried by streams is called alluvium. This becomes distributed evenly over the fan as the stream slowly sweeps back and forth, changing its position constantly.