As the river overflows its banks, the water spreads out, becomes shallow, loses its velocity, and drops much of its sediment load as the water moves out onto the fields of the floodplains. The process of a river flowing out of its channel and depositing sediments on its floodplain is called overbank deposition . The coarsest materials are deposited immediately adjacent to the stream channel and form low berms called natural levees . Humans also build levees along channels to control flooding. Otherwise, they are dry.
Ephemeral streams are most common in the arid and semi-arid regions of the western United States.
In contrast, perennial streams flow year around. Drainage basins refer to areas where a network of connected streams contributes water to another stream, lake, sea or ocean at the entrance of the drainage basin. Drainage patterns are often dendritic , which means that they resemble the veins in a leaf or the branches connected to the trunk of a tree. Precipitation contributes water to the streams in the basin. The water then flows through the connected streams and exits out of the basin into another stream, lake, sea or ocean.