WSP06041

• File Number: 8210.470
• Description: Pacific Southwest Interagency Committee
• State: CO
• Basin: Colorado Mainstem
• Water Division: 5
• Date: 10/1/1968
• Author: Unknown
• Title: Report of the Water Management Technical Subcommittee - Factors Affecting Sediment Yield and Measures for the Reduction of Erosion and Sediment Yield - October 1968
• Water Supply Pro - Doc Type: Report/Study

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The Pacific Southwest is made up of land with all classes of ground cover.
The high mountain areas generally have the most vegetation, while many
areas in the desert regions have practically none. The abundance of vege-
tation is related in a large degree to precipitation. If vegetative ground
cover is destroyed in areas where precipitation is high, abnormally high
erosion rates may be experienced.

Differences in vegetative type have a variable effect on erosion and sedi-
ment yield, even though percentages of total ground cover may be the same.
For instance, in areas of pinyon-juniper forest having the same percentage
of ground cover as an area of grass, the absence of understory in some of
the pinyon-juniper stands would allow a higher erosion rate than in the area
of grass.

Land Use

The use of land has a widely variable impact on sediment yield, depending
largely on the susceptibility of the soil and rock to erosion, the amount
of stress exerted by climatic factors and the type and intensity of use.
Factors other than the latter have been discussed in appropriate places in
this guide.

In almost all instances use either removes or reduces the amount of natural
vegetative cover which reflects the varied relationships within the environ-
ment. Activities which remove all vegetation for parts of each year for
several years or permanently are cultivation, urban development and road
construction. Grazing, logging, mining, and fires artifically induce per-
manent or temporary reduction in cover density.

High erosion hazard sites, because of the geology, soils, climate, etc., are
also of high hazard from the standpoint of type and intensity of use. For
example, any use which reduces cover density on a steep slope with erodible
soils and severe climatic conditions will strongly affect sediment yield.
The extent of this effect will depend on the area and intensity of use
relative to the availability of sediment from other causes. Construction of
roads or urban development with numerous cut and fill slopes through a large
area of widespread sheet or gully erosion will probably not cause a change
in sediment yield classification. Similar construction and continued dis-
turbance in an area of good vegetative response to a favorable climate can
raise yield by one or more classifications.

Use of the land has its greatest potential impact on sediment yield where a
delicate balance exists under natural conditions. Alluvial valleys of fine,
easily dispersed soils from shales and sandstones are highly vulnerable to
erosion where intensive grazing and trailing by livestock have occurred.
Valley trenching has developed in many of these valleys and provides a large
part of the sediment in high yield classes from these areas.