After a rainfall, water fills the stream channels called wadi  1. The rapidly flowing water cuts away the rocks of desert mountains and carries sediments created fan-shaped known as alluvial fans 2. Sometimes, the streams carry water into low areas in the desert plains and form into temporary lakes. The water that collects in these lakes either evaporates or seeps into the ground. Water erosion also creates big flat-topped hills known as mesas 3. and smaller flat-topped hills called buttes 4.