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. Drainage basins may be small and local or cover large areas of continents, such as the Mississippi River drainage basin. In turn, large drainage basins, like the Mississippi River's, may be divided into smaller drainage basins, such as those of the Missouri and Ohio rivers. Drainage divides are ridges or other areas that separate different drainage basins. They may range from a small ridge between two mountain valleys or continental divides, which may extend for thousands of miles.