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

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channel reach. Adjustments are made by the stream when the ability
to transport sediment is decreased or part of the sediment load is
deposited because of reduction in slope or absorption of flow into a
permeable channel bed.

Because of the complexities of the system involved in erosion on
hillslopes and channels in upland areas and the entrainment and trans-
port of the eroded material, accurate sediment routing and definition
of sediment source areas is difficult. In arid areas particularly,
where storms generally are widely scattered and of small areal extent,
the eroded sediment is often transported only a short distance and
redeposited in a single event. Therefore, the relation of sediment
yield rates from small upland areas to larger areas downstream is too
complex to evaluate in most cases.

The transportation of sediment by streamflow is complex because of the
many variables involved. Fine particles are transported in suspension
and may be moved far downstream. Coarse particles may be transported
momentarily in suspension but are usually rolled along the streambed.
Other important factors in sediment transport are the supply that is
available for movement, the sizes and density of particles, channel
geometry, characteristics of the flow, and quality of water.

Most of the sediment that is eroded from upland areas eventually makes
its way to the valley floors and channel flood plains before it is
deposited. Some of the sediment is deposited enroute to these down-
stream points in reservoirs and other water regulating structures or
on alluvial fans where stabilization by vegetation occurs. It is
important to recognize, however, that any deposit of unconsolidated
sediment probably will be transitory. A change in climate, land use
or vegetational cover may regenerate the sedimentation cycle and
transport the sediment farther downstream.

METHODS INVESTIGATED

The methods available to estimate on-site erosion and sediment yield
from small watersheds were determined from an extensive literature
search and by contacting agencies with water resources and sedimentation
research programs in the Pacific Southwest area to ascertain the procedures
they use. In spite of a large bibliography of published material on
erosion and sedimentation, the number of methods that are germane to
this report was narrowed to 12. Six of the methods provide estimates
of on-site erosion and six provide estimates of sediment yield in a
stream draining a small natural watershed.

A plan of evaluation of the selected methods was developed in order
to accomplish the objectives of the task force. The work plan consisted
of the following steps: (1) summarize the pertinent characteristics
of each method, (2) describe the origin and development of each method,
including the geographic location of research sites, and (3) test the

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