Earth Science: Streams
Streams are the channelized flow of surface water, and the single most
effective geologic agent responsible for the landforms we
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
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.
Evolution of a floodplain.
Meanders in the Sweetwater River, Wyoming.
Formation of oxbow lakes.
Landsat view of braided stream Brahmaputra r., India.
Entrenched meanders in the San Juan r., Utah.
Flood plain terraces, Snake r., Wyoming.
Landsat view of the Mississippi delta.
Last modified: Wed Mar 1 13:38:44 CST 2000