SEDIMENT BUDGETS FOR TWO SEMI-ARID WATERSHEDS, RIO PUERCO BASIN, NEW MEXICO

Three major channels have cut and filled the Rio Puerco valley in the past 3,000 years. Recent surveys indicate that the Rio Puerco channel is now in a cycle of aggradation, which is independent of a change in the base level of the Rio Grande. This raises questions about the source(s) of sediment and processes that govern aggradation.

To determine possible sediment sources in the Rio Puerco basin, a sediment budget was constructed for two subbasins of the Rio Puerco, Volcano Hill Wash (9.30 km²) and Arroyo Chavez (2.28 km²), from 1995-98. Sediment yields and sediment concentrations were measured with sediment traps and at a streamflow-gaging station located at the outlet of each subbasin.

The average annual sediment yield measured at the streamflow-gaging station at Arroyo Chavez (981 tonnes/km²/yr) is more than twice the sediment yield of Volcano Hill Wash (405 tonnes/km²/yr). Similarly, the discharge-weighted sediment concentration measured at the streamflow-gaging station at Arroyo Chavez (92,500 mg/L) is more than twice the concentration at Volcano Hill Wash (34,000 mg/L). Averaging sediment yields from all sediment traps indicates the sediment yield for Arroyo Chavez (0.683 kg/m²/year) is more than twice the average sediment yield for all traps at Volcano Hill Wash (0.274 kg/m²/year). Accounting of sources in each basin shows that 57% of the sediment transported out of Arroyo Chavez is from channel erosion (bed and bank) and 43% is from upland sheetwash erosion. In Volcano Hill Wash, 32% is from the channel and 68% is from sheetwash erosion.

Differences in land-use factors could contribute to the higher erosion and sediment yields in Arroyo Chavez compared to Volcano Hill Wash. Although both subbasins predominately are used for grazing, Arroyo Chavez is considered in poor range condition and Volcano Hill Wash is in good condition. Arroyo Chavez also contains gas pipeline activity, dirt roads, and a higher livestock density. Visually, the basin is gullied with many areas of bare ground. The larger contributions from channel erosion in Arroyo Chavez may reflect human disturbances.