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<br /> <br />Frontal storms in whi~h periods of moderate to high intensity precipitation <br />occur can produce the highest sediment yields within the Southwest. In <br />humid and subhumid areas the impact of frontal storms, on sediment may be <br />greatest on upland slopes and unstable geologic areas where slides and other <br />downhill soil movement can readily occur. <br /> <br /> <br />- 5 - <br /> <br />., <br /> <br />N <br />00 Snow appears to have a minor effect on upland slope erosion since raindrop' <br />-..] impact is absent and runoff associated with snow melt is generally in <br />~ 'resistant mountain systems. <br /> <br />" <br /> <br />Convective thunderstorm activity in the Southwest has its greatest influence <br />on erosion and sedimentation in Arizona and New Mexico and portions of the <br />adjoining states. High rainfall intensities on low density cover or easily <br />dispersed soils produces high sediment yields. The average annual sediment <br />yield is usually kept within moderate 'bounds by infrequent occurrence of <br />thunderstorms in anyone locality. ' <br /> <br />High runoff of rare frequency may cause an impact on average annuai sediment <br />yield for a long period of time in a watershed that is sensitive to erosion, <br />or it may have little effect in an insensitive watershed. For example, sedi- <br />ment that has been collecting in the bottom of a canYon and on side slopes <br />for many years of low and mode1:'ate flows may,pe swept 'out during the rare <br />event, creating a large change in the indicated sediment yield ,rate for, the <br />period of record. <br /> <br />In some 'areas the action of freezing and thawing becomes important in the <br />erosion process. Impermeable ice usually forms in areas of fine textured <br />soils where a supply of moisture is available before the advent of cold <br />weather. Under these conditions the ice often persists throughout the winter <br />and is still present when'the spring thaw occurs. In some instances water, <br />tends to rtin over the surface of the, ice and not deta,ch ,soil particles, ,but <br />it is possible for the ice in a surface layer to thaw during a warm period <br />and .create a very erodible situation. Spring rains with ice at shallow depth <br />may wash away the loose material on'the surface. <br /> <br />TOPoj1;raphv <br /> <br /> <br /> <br />In some areas of the Pacific Southwest, particularly ,those underlain by marine <br />shale,' freezing and thawing alters the texture of soil near the surface, and <br />thus changes the infiltration characteristics. These areas generally 'do not <br />receive enough snow or have cold enough temperatures to build a snow pack for <br />spring melt. Later in the year soil in a loosened condition is able to <br />absorb a large part of the early rainfall. As rains occur during the summer,' <br />the soil becomes compacted on the surface, thus allowing more water to run <br />off and affording a greater chance for erosion. <br /> <br />Watershed slopes, relief, floodplain development, drainage patterns, orient- <br />ation and size are basic items to consider ,in connection with topography. <br />However, their influence is closely associated with geology, soils, and cover. <br />