GEO 101 Lecture Notes - Lecture 28: Drainage Divide, Downhill Creep, Water Cycle
24 Jun 2018
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Regional Erosion
Slope erosion. Early theories suggested that mountains gradually eroded down to form
plains over very long stretches of time—the sides and tops of the mountains were
eroded to gentler slopes at similar rates, gradually losing elevation and approaching
base level. Today it is more accepted that many slopes erode according to parallel
retreat. Instead of eroding to gentler angles over time, slopes maintain their original
steepness as they erode progressively back from river channels
The climate, rock type, and rock structures are the most important factors in slope
erosion. As previously noted, rocks of different compositions weather at different rates—
harder rocks form steeper slopes and generate coarser grained, more angular talus ‐
slopes that are resistant to weathering and erosion. Sand and clay slopes erode more
easily and are less steep. Certain climates accelerate soil creep and chemical
weathering, which result in more rounded topography. Rock structures such as folds,
bedding planes, and faults can also affect the slopes that result from weathering.
Drainage patterns. A stream and its tributaries form geometrical arrangements
called drainage patterns. A drainage pattern can be greatly influenced by the geologic
formations through which it passes. A treelike dendritic pattern develops in a rock type
that erodes uniformly, such as granite. A radial pattern, which resembles the spokes on
a wheel, occurs when the streams originate on the flanks of conical mountains. A trellis
pattern consists of parallel main streams with short tributaries on either side that form in
areas of tilted sedimentary rocks that create parallel ridges and valleys. Bedrock that is
regularly fractured or jointed in 90 degree angles can create‐ rectangular patterns, which
have distinctive right angle bends.‐
Types of Water Flow
The hydrologic cycle is the constant circulation of the earth's water through
precipitation, evaporation, and transpiration (the release of water into the atmosphere
by plants). It is the continuous exchange of water between the atmosphere, land, and
ocean. Running water is the most active landscape transforming agent on the earth's ‐
surface. Waterways erode, transport, and deposit rock and sediment to produce
landforms such as canyons, valleys, deltas, alluvial fans, and floodplains.
Streams (any flow of water within a natural channel regardless of size) are the most
important kinds of channel flow that affect landscapes. A stream's headwaters are
where the stream originates, usually in the higher elevations of mountainous terrain.
The stream flows downhill and across lower elevations to its terminus, where it enters
another stream, lake, or ocean. This terminus is called the mouth of the stream.
The stream is often flanked on both sides by a flat floodplain that is created when
periodic flooding deposits mud and silt over extensive, low lying areas. Flooding results ‐
when a stream's flow is increased and exceeds the capacity of the stream
channel. Water sometimes moves overland during heavy storms as a sheetwash, a thin
layer of unchanneled water. Sheetwashes typically occur in arid climates or where the
ground is saturated and cannot accept any more water. Eventually sheetwash flow
forms small channels called rills;rills join to form larger temporary streams.
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Document Summary
Early theories suggested that mountains gradually eroded down to form plains over very long stretches of time the sides and tops of the mountains were eroded to gentler slopes at similar rates, gradually losing elevation and approaching base level. Today it is more accepted that many slopes erode according to parallel retreat. Instead of eroding to gentler angles over time, slopes maintain their original steepness as they erode progressively back from river channels. The climate, rock type, and rock structures are the most important factors in slope erosion. As previously noted, rocks of different compositions weather at different rates harder rocks form steeper slopes and generate coarser grained, more angular talus slopes that are resistant to weathering and erosion. Sand and clay slopes erode more easily and are less steep. Certain climates accelerate soil creep and chemical weathering, which result in more rounded topography.
