Abstract

Mechanical analyses of representative samples indicate that alluvial deposits accumulated in the Middle Rio Grande Valley, during the period 1936-41, had an average composition of approximately 43% sand, 41% silt, and 16% clay according to the Wentworth size grades. The median grain size was about 0.054 mm., which is in the coarsest silt size of the Wentworth classification or in the very fine sand size of the soils classification of the Department of Agriculture. Corresponding determinations of the sediment density (dry weight per unit volume) indicate an average density of about 86 pounds per cubic foot for the median grain size of all the deposits, about 85 pounds per cubic foot for the overbank flood-plain deposits, and 100 pounds per cubic foot for the flyer-bed sediment. The results lend support to the use of sand sizes as criteria of the sources of all sediment in the Valley. The textural data also indicate that sand accumulation along the Rio Grande is due to excessive supply from tributary sources rather than lag accumulation; that channel avulsions and "splay" deposits of sand on the flood plain are major factors in the river regime; and that extension of the textural studies might provide a useful measure of the relative importance of different erosional processes. In combination with other available data, the density determinations provide a basis for estimating the entire rate of sediment output, or net erosion, from the drainage basin above Elephant Butte Reservoir, which is at the lower end of the Middle Valley. This rate is indicated as 0.7 acre-foot per square mile for the period 1936-41, equivalent to an average rate of surface lowering of 1 inch in about 75 years. Other studies indicate that this 5-year rate is probably somewhat lower than the average for a period of 15 to 25 years.

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