SEDIMENTATION IN THE MIDDLE RIO GRANDE VALLEY, NEW MEXICO

Abstract

Surveys have shown sediment accumulation at a rate of 12,000 acre-feet per year, or 22.5 million tons, from 1936 to 1941 in the Middle Rio Grande Valley of central New Mexico. Throughout most of the upper 133 miles of the Middle Valley, the Rio Grande is confined within a levee-bordered floodway, which was aggraded at a rate equivalent to 0.08 foot per year, or about 1 foot in 12 years. This was due to deposition on the berms between the levees and river, chiefly during major floods in 1937 and 1941, for there was practically no net change in average river-bed elevation through the floodway. The relation between berm and river-bed deposition is believed to be temporary, resulting from the scouring effect of a major flood shortly before the 1941 surveys, for comparisons of water-surface elevations indicate river aggradation from 1917 to 1936 at about the same rate as the 1936–1941 floodway aggradation. About 55 per cent of the 1936–1941 sedimentation was in the lower 14 miles of the valley, between the lower end of the floodway and the head of Elephant Butte Reservoir. This sediment concentration was due chiefly to headward growth of the reservoir delta, several channel avulsions that happened to occur during the short period of record, and perhaps some influence of the floodway levees in confining flood waters and forcing transportation of sediment into the lower part of the valley. The sedimentation and river aggradation are serious problems affecting the future of irrigation, drainage, and flood-protection works essential for the productivity of more than 100,000 acres of agricultural land, and increasing flood hazards for about half the city of Albuquerque, many smaller communities, parts of a transcontinental railway and one of its major branches, and several major highways. The Middle Rio Grande may have been aggrading for centuries, but the rate of sedimentation is believed to have increased greatly since about 1870 as a result of arroyo erosion in the alluvial valleys of many intermittent tributaries. The arroyo erosion apparently was caused by depletion of the natural vegetation and trampling of trails by cattle and sheep. Some combination of detention reservoirs and channel improvements may provide temporary relief, perhaps for many years, but a successful long-term remedial program evidently will require stabilization of the eroding arroyos. Unfortunately it has not yet been demonstrated that effective measures for arroyo stabilization are practicable and economically feasible.