File:Dust storm off Alaska (MODIS 2016-10-26).jpg

English: Strong winds kicked up a spectacular dust storm off of Alaska in late October 2016. The Moderate Resolution Imaging Spectroradiometer (MODIS) aboard the Aqua satellite captured this true-color image of northerly winds blowing dust from the Copper River Delta on October 24.

The gray, grainy material pouring off the Copper River Delta is an iron- and feldspar-rich dust which has been created by the grinding of glaciers against underlying bedrock. The silty powder is also called “glacial flour” or “rock flour”. Glacial flour can find its way into rivers and lakes, especially during springtime snow melt when the water levels are high. Glacial flour in flowing rivers can be carried far downstream before settling out on dry land when river levels drop. Once dry, the flour can be easily lifted by winds and carried long distances.

The Copper River rise from the Copper Glacier located on the northeast side of Mount Wrangell in the Wrangell Mountains positioned in the interior of Alaska. The river drains an area about the size of the state of West Virginia as it courses over about 290 miles (467 km) – a drainage basin that includes several glaciers. The river forms a broad delta, about 50 mi (80 km) wide, where the waters flow into the Gulf of Alaska. Glacial flour accumulates along the course of the river, as well as in the delta, making the valley a rich source of spectacular dust storms.

Dust events in this region are most common in the fall, when river and lake levels in south central Alaska are at their lowest and strong winds are common. Early spring and late winter are also times when dust activity is elevated.