9.2: Stream Drainage Basins and Patterns

Stream Drainage Basins and Patterns

The drainage basin Links to an external site. of a stream includes all the land that is drained by one stream and all of its tributaries Links to an external site.. Find out more about the drainage basin or watershed you live in by visiting the Environmental Protection Agency’s How’s My Waterway Links to an external site.?

The higher areas that separate drainage basins are called drainage divides Links to an external site.. For North America, the Continental Divide Links to an external site. in the Rocky Mountains separates water that drains west to the Pacific Ocean from water that drains east to the Gulf of Mexico or Atlantic Ocean (Figure 9.3).

The continental divide, which is oriented north-south roughly along the Rocky Mountains, bisects the country into two major drainage basins, one to the east and the other to the west. — **Figure 9.3:** Water that falls to the west of the continental divide will ultimately flow into the Pacific Ocean, whereas water that falls to the east will flow into the Atlantic Ocean or Gulf of Mexico. (CC-BY 4.0; Chloe Branciforte via Google Earth)

As water flows over rock, it is influenced by it. Water follows the path of least resistance; this means it will weather and erode softer rock first, rather than more resistant rock. This can result in characteristic patterns of drainage Links to an external site. (Figure 9.4). Some of the more common drainage patterns include:

Dendritic Links to an external site.: this drainage pattern indicates uniformly resistant bedrock that often includes horizontally layered sedimentary rocks. Since all the rock is uniform, the water is not attracted to any one area, and spreads out in a branching pattern, like the branches of a tree.
Trellis Links to an external site.: this drainage pattern indicates alternating resistant and non- resistant bedrock that has been deformed (folded) into parallel ridges and valleys. The water erodes valleys in the softer rock, and appears much like a rose climbing on a trellis in a garden.
Radial Links to an external site.: this drainage pattern forms as streams flow away from a central high point, such as a volcano, resembling the spokes in a wheel.
Rectangular Links to an external site.: this drainage pattern forms in areas in which rock has been fractured by jointing or faulting which created weakened zones in the . Streams erode the weakened, less resistant rock and create a network of channels that make right-angle bends as they follow the intersecting fracture pattern. This pattern will often look like rectangles or squares.
Deranged Links to an external site.: this drainage pattern does not follow the rules. It consists of a random pattern of stream channels characterized by irregularity. It indicates that the drainage developed recently and has not had time to form one of the other drainage patterns yet.

Five major drainage patterns and their overall appearance in map-view. — **Figure 9.4:** Drainage patterns. (CC-BY-SA 3.0; Corey Parson Links to an external site., modified by Chloe Branciforte)