WSP01538

• File Number: 8210.470
• Description: Pacific Southwest Interagency Committee
• State: CO
• Basin: Colorado Mainstem
• Water Division: 5
• Date: 4/1/1974
• Author: Unknown
• Title: Report of the Water Management Technical Subcommittee - Erosion and Sediment Yield Methods - April 1974
• Water Supply Pro - Doc Type: Report/Study

by mechanical treatment, cultivation, vegetation manipulation, or
structural regulation of streamflow has a marked influence on the
sedimentation cycle. The processes and products of sedimentation are
represented in the entire geologic column but the part of the cycle of
primary interest here is modern sedimentation influenced to a large
degree by man's activities.

Erosion by water begins with the intia1 detachment of soil or weathered
rock material on upland areas. This kind of erosion may be divided
into sheet and rill erosion and channel erosion, which includes gullying.
These basic processes are the first step in the movement of sediment
from upstream source areas to downstream locales of deposition.

Sheet erosion may be defined as the removal of soil and weathered rock
material as a thin sheet by overland flow that is not concentrated
in well-defined channels. The eroded material being transported results
from impact energy as individual raindrops intersecting the land dislodge
sediment and form the shear force produced by the.water moving over the
land surface. It is difficult to differentiate between sheet and
rill erosion in the field because runoff will tend to concentrate
quickly in small rills on irregular surfaces. Therefore, in less
advanced stages, sheet erosion is an intangible factor in the evaluation
of soil loss or sediment yield from any watershed. Without continued
accurate quantitative measurements along established ranges, the amount
of sheet erosion is generally too small to be observed. Criteria that
have been used to estimate the approximate rate of sheet erosion are
such features as pedestaled vegetation, shallow rills, and soil
characteristics. Such evidence may indicate serious erosion, but the
extent and rate cannot always be determined precisely.

The relation between sheet erosion and rock type is variable. Where
bedrock is covered by a thick mantle of weathered rock and residual
soil, sheet erosion is generally more severe than in areas where the
bedrock is at or near the surface. However, sheet erosion can be
severe on exposed bedrock, such as shale, which has a low inherent
resistance to erosion. Also, the severity of sheet erosion is governed,
to a large degree, by the type and density of the plant cover and, to
a lesser extent, by the characteristics of the weathered mantle such
as grain size and infiltration capacity.

Runoff from hi11s10pes in upland areas concentrates in rills and small
channels. These channels increase in size in a downstream direction
as the drainage network enlarges. Channel erosion is the detachment
of sediment from the bed and banks of stream channels by fluvial processes.
Channel erosion is controlled by many factors such as the bed slope,
velocity of flow, suspended-sediment load being carried by the stream,
cohesiveness of bed and bank material, plant cover on banks, and particle
size of sediment available for transport in the stream. Therefore,
channel erosion generally is quite variable from place to place in any

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