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. The most famous continental divide is in the Rocky Mountains and separates streams that flow east into the Gulf of Mexico from those that flow west into the Pacific Ocean. However, there are also other continental divides in North America. For example, continental divides in North Dakota and Minnesota separate the Mississippi River drainage basin, which empties into the Gulf of Mexico, from drainage basins to the north, which empty into Hudson Bay, Canada or the Great Lakes. Until the building of the Aswan Dam in Egypt, the Nile River annually flooded.