• The area drained by a stream and its tributaries is a drainage basin or watershed
• The Mississippi River drainage basin covers approximately half of the conterminous U.S.
• Drainage basins are separated by high ground termed a divide
• Drainage patterns are governed by the underlying geology of a region

pproximately two thirds (68%) of Earth's land surface contains rivers that drain to the ocean.  Land that is permanently covered by ice (Antarctica) or that is characterized by hot, dry deserts typically lack stream systems. 

Diagram of drainage basins for major tributary streams to the Mississippi River. Image modified from a map at the National Watershed Network.

A drainage basin or watershed is the area drained by a stream and its tributaries. The size of the drainage basin typically increases as the size of the stream increases. The drainage basin for the Mississippi River occupies approximately half the land area of the U.S. The two principal tributaries of the Mississippi are the Missouri and Ohio rivers. The drainage basins for each of these (and other) rivers are included within the larger Mississippi drainage basin.

Illustration of drainage basins and drainage divides. Click on image for an expanded view.

The Mississippi River is separated from streams that drain to the west by a drainage divide represented by the Rocky Mountains. Streams west of the Rockies flow to the Pacific Ocean or Gulf of California. Streams in the eastern U.S. flow to the Gulf of California or the Atlantic Ocean. 

In Ohio (see below) a drainage divide separates streams that drain northward to the Great Lakes from streams that flow south to the Ohio River. The city of Akron in northeast Ohio straddles the divide between the Cuyahoga and Tuscarawas Rivers. The Cuyahoga River is part of the Great Lakes Basin and drains to the north. The nearby Tuscarawas River is a tributary of the Muskingum River. Both rivers are in the Ohio River watershed, a major tributary of the Mississippi River.

Selected Ohio river basins. Streams north of the drainage divide flow to the Great Lakes and ultimately through the St. Lawrence seaway to the Atlantic Ocean. Streams south of the divide flow to the Ohio River which joins the Mississippi River and flows to the Gulf of Mexico.

Drainage Patterns
The pattern of drainage in a basin is largely determined by the underlying geology. Streams typically follow the path of least resistance, forming valleys where rock is most readily eroded or following the steepest slope gradient. Consequently, stream patterns often provide clues to the geology of the drainage basin. Dendritic drainage is characteristic of areas where the geology is relatively uniform, for example where rock layers are horizontal. Dendritic patterns are typical of Ohio streams and of much of the Colorado river basin (see map). Streams intersect with a characteristic V-pattern in map view in dendritic drainage systems. The tip of the V points downstream.

Streams intersect at right angles in both trellis and rectangular drainage patterns. Trellis drainage is characteristic of areas with alternating parallel valleys and ridges (e.g central Pennsylvania). Streams occupy the valleys, flowing parallel to the ridges but occasionally cutting across the geological grain at water gaps.  Rectangular patterns distinguish regions where the bedrock exhibits well developed joints or fractures. The streams exploit the fractures as lines of weakness. Finally, radial drainage patterns are typically found on the slopes of volcanoes, where streams flow downslope, parallel to the slope gradient.

The four principal types of drainage pattern are related to the underlying regional geology.