Erosion and Sedimentation in the Upper Gila Drainage, A Case Study

Abstract

The upper Gila River in Arizona and New Mexico contains extremely diverse geology and soils. One geological formation that is somewhat unique to the Southwest and the upper Gila drainage is the Gila Conglomerate formation. In New Mexico, this conglomerate is extensive on the main Gila River drainage, accounting for over 35 percent of the main basin area. A case study was done on the 22,580 hectare (55,793 acres) Lake Roberts Watershed to assess the current sedimentation problem and its sources. This study revealed interesting patterns of lake surface area changes with volume changes of the original 28.3 hectare (70 acres) man-made reservoir over the last 12 years. Surface area reduction (19%) has progressed at a rate over twice that for volume reduction (9%). The source of the problem stems primarily from soils derived from highly sensitive Gila Conglomerate. The watershed is not uncharacteristic of the unique geology and soils typical of the upper Gila drainage and may furnish insight into sediment production and sources for much of the Gila headwater drainage in New Mexico.