Debris-Covered Hillslopes of the Southern Arizona Desert: Consideration of Their Stability and Sediment Contribution

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Abstract
The various processes that affect hillslopes in the Sonoran Desert of southern Arizona vary considerably in relative importance, depending upon elevation (and hence climate), angle of slope, and the nature and weathering properties of the bedrock. Hillslopes on volcanic bedrock generally have fixed surfaces, whereas hillslopes on granitic bedrock have mobile surfaces. The difference is related to the method and rapidity of production of boulders from the bedrock mass. Large boulders on hillslopes below about a 2,000-foot elevation show evidence of past episodes of case-hardening and deposition of desert varnish, whereas the fragments on slopes above about a 3,000-foot elevation have undergone more rapid or more prolonged mechanical weathering, probably by frost action and chemical weathering, that has destroyed most of the case-hardening and desert varnish. Hillslopes of less than 28.5° on both granite and volcanic rocks are highly sTable, whereas those between 28.5° and up to 36.0° are increasingly unsTable, and the upper limit of debris-covered hillslopes is approximately the angle of repose of the debris. Debris-covered hillslopes steeper than about 36° are rare and require some additional agency, seldom present under desert conditions, to maintain the surface cover. Steeper, bare-rock slopes are common, however. The two stability limits, 28.5° and approximately 36.0°, are related to the frictional properties of the rock itself. An effort to find a statistical correlation between slope angle and fragment size was largely unsuccessful, and it is suggested that the concepts of the boulder-controlled slope and the repose slope be abandoned.

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