When glaciers grind against underlying bedrock, they produce a silty powder with grains finer than sand. Geologists call it glacial flour or rock flour. This iron- and feldspar-rich substance often finds its way into rivers and lakes, coloring the water brown, grey, or aqua. When water levels are low, the flour can dry out on riverbanks and deltas and become dust for winds to lift into the air. The Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA's Aqua satellite captured this image of northerly winds blowing dust from the Copper River Delta over the Gulf of Alaska. Dust events are most common in this area in the fall, when river and lake levels in south central Alaska are at their lowest. Another period of high dust activity occurs in late winter and early spring.