At the margins, streams run out of the mountains and are augmented by freshwater springs in the foothills. These rivers fan out as they flow toward the center of the basin. In this satellite image, those oases appear deep greenish-brown in the west and southwest of the desert, while a river of rippling dust blows across the northern part of the desert and out of sight at image right. Frequent dust storms in April and May can blanket the whole region for days at a time, sometimes blocking out the sun. The dust storms can spread for hundreds, even thousands, of kilometers, and are a health and safety problem for people in their path.