Chapter 2B Main Ideas

1. Drainage Basins

A drainage basin is the land area from which water flows to supply water for a stream. Drainage basins can be discussed at different scales, such that there can be many smaller drainage basins within an overall larger drainage basin.

2. Controls on Erosion in Stream Systems

A stream’s ability to erode is based on its velocity (speed of flow) which is controlled by gradient (slope) and discharge (volume of flow). Streams are divided into three zones based on the changing gradient from stream head to mouth: headwater, transfer, and depositional. Steeper areas create higher velocity. Discharge increases as more water is added to a stream, which increases velocity. If discharge is constant, velocity will increase if the stream is forced through a narrow passage.

3. Stream Features and Deposits

Changes in stream velocity create distinct features along a river. Alluvial fans develop where streams flow out of mountains onto valleys. Along meandering streams, cutbanks, pointbars, floodplains, and oxbow lakes develop. At the mouths of rivers, deltas form.

4. Colorado River and Irrigation

The western U.S. is dry because of the orographic effect. This dryness has been exacerbated because of climate change leading to prolonged drought throughout the region. This drives an increased need for irrigation water, much of which comes from the Colorado River in this area. The Colorado River Compact was established to set limits on how much water could be used by the Upper Basin, Lower Basin, and Mexico, however, the limits set are higher than the amount of water available. This coupled with increased water usage is leading to water shortages in the basin.

5. Water Diversions and Dams

Dams and water diversion projects are important for supplying irrigation water from river systems; however, they have many drawbacks including displacing people, changing local climates by changing evaporation, raising water temperatures, changing sediment loads in a river, changing riverbed erosion, and fragmenting river ecosystems. If diversion projects are large, this creates decreased water volumes downstream and rivers may stop flowing to their deltas, like the Colorado River.