Strahler’s Model of Slope Development

The work of Strahler is statistical in nature. His work is based on the data collected from fieldwork in parts of California. Strahler (1950) collected data “to determine

1. If differences in underlying rock types are associated with differences in slope angle.
2. If differences in directional exposure to sunlight and other meteorological factors produce differences in slope angles, and
3. If slopes decline in angle when left to weathering and erosion processes and not accompanied by basal erosion and removal”.

He conducted measurements of maximum angles attained by slope, carefully identified the area, and ensured it should have uniformity in terms of climate, vegetation, relief and tectonic history. Lithological factors, however, were not the same.

He calculated the mean maximum slope angle for the study area, then assessed the deviation of slope from the mean slope by comparing the data of slope collected at different points in the study area.

Strahler argued that if a large number of slopes show very little variation from the mean slope, it means that the slopes have developed at approximately the same angle for the reason that this is the angle allowing the steady and efficient removal of the slope debris by slumping, creep and wash. Such slopes are in a delicate state of equilibrium (Small,1978).

According to Strahler (1950), “under the equilibrium, slopes maintain an equilibrium angle proportional to the channel gradient of the drainage system and are so adjusted as to permit a steady state to be maintained by the process of erosion and transportation under prevailing conditions of climate, vegetation, soils, bedrock and initial relief”.

Thus one can infer that the equilibrium slopes are governed by different slope controlling factors, and change in any of the factors can cause readjustment of the equilibrium angle.

Strahler, during his field observation, also studied the relationship between valley side slopes and the stream gradient and noted that with the reduction of landmass, there is a reduction in stream gradient and slopes. They gradually regrade towards the maintenance of equilibrium.

He confirmed the correlation between slope angle and channel gradient (slope adjusts in proportion to the debris obtained from the valley side slopes); where the valley side slope is steep, it will have a steep channel slope, and where it is gentle, it will have a gentle channel slope.

However, there are exceptions to this rule in that the side slopes do not steepen with the increase in channel gradient very near the head of streams (Sparks,1986).

Strahler also noted that a bare slope would contribute a greater amount of load than a vegetated slope at a given angle. This will result in a steeper channel gradient beneath the bare slope than that below the vegetated. This example displays how both the angle of the stream channel and valley side slope modify with the change in controlling factor (i.e. vegetation cover).

Through careful measurements of slopes, Strahler discovered that where the river was closer to the foot of the slope it formed a steeper slope because of debris removal. But when the stream was away from the slopes, they were protected from the basal cutting and had lower angles.

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