GEO 101 Lecture Notes - Lecture 27: Headward Erosion, Downcutting, Mass Wasting
24 Jun 2018
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Stream Valleys
The erosion and transport of rock and sediment by a stream defines the shape and
extent of its valley. V shaped valleys and wide valleys with flat floors are the most ‐
common varieties.
Downcutting. A valley is the result of downcutting, whereby a stream's channel
erodes directly downward. As downcutting continues, erosion and mass wasting begin
to work on the exposed, vertical sides of the channel, eroding them into slopes and
widening the valley
Slot canyons, vertical walled rock canyons where mass wasting processes have been ‐ ‐
very limited, are a common feature in the western United States
As a stream flows downslope and gains more water from tributaries, the valley becomes
wider because of greater mass wasting. Downcutting proceeds until the base level is
reached—the elevation of the most horizontal flow and lowest velocity. For streams that
empty into the ocean, base level is essentially sea level. Base level for continental
streams is generally the lowest elevation of the valley.
Ungraded and graded streams. An ungraded stream is one that is still actively
downcutting and smoothing out its irregular gradient through erosion. It is characterized
by rapids and waterfalls. In contrast, a graded stream has smoothed out its longitudinal
profile to resemble a smooth, concave upward curve. How long it takes a stream to ‐
become graded is influenced by sediment load, which affects the rates of erosion and
downcutting. Dams also have a major impact on grading by reducing stream flow and
sediment load.
Headward and lateral erosion. Valleys are further developed by headward erosion
and lateral erosion. Headward erosion results when a valley is extended upward above
its original source by gullying, mass wasting, and sheetwash flow. Lateral
erosion occurs when the stream meanders or braids back and forth across its valley
floor or channel, undercutting and eroding its banks. This results in mass wasting of the
gradually more unstable slopes and forms a wider floodplain.
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Document Summary
The erosion and transport of rock and sediment by a stream defines the shape and extent of its valley. V shaped valleys and wide valleys with flat floors are the most common varieties. A valley is the result of downcutting, whereby a stream"s channel erodes directly downward. As downcutting continues, erosion and mass wasting begin to work on the exposed, vertical sides of the channel, eroding them into slopes and widening the valley. Slot canyons, vertical walled rock canyons where mass wasting processes have been very limited, are a common feature in the western united states. As a stream flows downslope and gains more water from tributaries, the valley becomes wider because of greater mass wasting. Downcutting proceeds until the base level is reached the elevation of the most horizontal flow and lowest velocity. For streams that empty into the ocean, base level is essentially sea level. Base level for continental streams is generally the lowest elevation of the valley.
