Earth Science: Streams

Streams

Streams are the channelized flow of surface water, and the single most effective geologic agent responsible for the landforms we see.

Stream downcutting is followed by mass wasting from the sides of the canyon so that a stream valley is formed.

Rivers and tributaries each drain a portion of land known as a drainage basin, shown below for the Mississippi river.

Water flows downhill from stream head in the highlands to stream mouth at a lake or (usually) the ocean. The drop in elevation with distance is called the stream gradient. The stream gradient is steep near the stream head, and shallow toward the mouth.

Floods occur when a stream is overloaded with more water than it can carry within its banks. The excess water spills into the stream's floodplain. In the natural (no humans) state, floods are common, and sometimes occur yearly.

Downtown Davenport, July 1993.

Landsat images of the Mississippi River at St. Louis, Missouri, in normal and flood stages.

Artifical controls on flooding:

Dams can store excess water in flood times. The downsides are (1) they can't handle big floods, (2) they become filled with sediment, decreasing their capacity, (3) agriculture suffers if nutrient-rich flood waters are cut off, as in Egypt, (4) dams can break, causing an even worse flood.
Levees are raised embankments that contain flood-stage rivers. The downsides are (1) sediment collects in the river-bottom so that the levees need to be built higher or the river channel dredged, (2) a really good levee can cause flooding upstream as flood waters bottleneck at the levee.
Channelization (deepening and straightening of river channels) allows a greater volume of water to flow.
Urbanization, with its accompanying asphalt and concrete, provides no temporary storage for precipitation, and this increases the potential for floods. Wetlands store large amounts of water, and their destruction likewise increases danger from floods. Any structures built in the floodplain decrease the volume available for flood waters, so that floods rise higher in the presence of many buildings.

Sequence of stream erosion.