Why does the White River add silt to the Yukon River?

The White River adds silt to the Yukon River because it is a glacier-fed stream that drains the north slopes of the St. Elias Mountains.1

As glaciers move, they erode the underlying bedrock, producing a fine-grained sediment known as rock flour. This sediment is transported by glacial meltwater into the river system. Because the White River originates from these mountainous, glaciated regions, it carries this high concentration of suspended material into the Yukon River, contributing to the river's sediment load.21