Comment: 02:48 - 03:46 (00:58)
Source: Annenberg/CPB Resources - Earth Revealed - 20. Running Water II: Landscape Evolution
Keywords: "Douglas D. Rhodes", river, valley, geologist, "mass wasting", sediment, stream, erosion, slope, "base level", degradation, "flow velocity", sea, "sea level"
Our transcription: The connection between a river and its deep wide valley is not an obvious one.
At one time, valleys were thought to have formed independently of the rivers which flow through them.
Today geologists are well aware that valleys usually form by the down-cutting of running water, combined with the mass wasting of slopes.
As a river cuts its channel deeper, it carries away sediment fed to it from surrounding hillsides.
Now there are limits to how deep that a stream can erode its valley, and those limits come up several kinds, which we generally refer to as "base level."
The ultimate base level, a grand base level is sea level.
Streams don't degrade their valleys below the level of the sea.
So we don't find great canyons arcing down to the ocean filled by water which is flowing back in from the sea.
The stream as it approaches the sea level loses velocity and, therefore, loses ability to erode.
Geology School Keywords