MODERN SEDIMENT YIELD COMPARED TO GEOLOGIC RATES OF SEDIMENT PRODUCTION IN A SEMI-ARID BASIN, NEW MEXICO: ASSESSING THE HUMAN IMPACT

In the semi-arid Arroyo Chavez basin of New Mexico, a 2·28 km2 sub-basin of the Rio Puerco, we contrasted short-term rates (3 years) of sediment yield measured with sediment traps and dams with long-term, geologic rates (~10 000 years) of sediment production measured using 10Be. Examination of erosion rates at different time-scales provides the opportunity to contrast the human impact on erosion with background or geologic rates of sediment production. Arroyo Chavez is grazed and we were interested in whether differences in erosion rates observed at the two time-scales are due to grazing. The geologic rate of sediment production, 0·27 kg m−2 a−1 is similar to the modern sediment yields measured for geomorphic surfaces including colluvial slopes, gently sloping hillslopes, and the mesa top which ranged from 0·12 to 1·03 kg m−2 a−1. The differences between modern sediment yield and geologic rates of sediment production were most noticeable for the alluvial valley floor, which had modern sediment yields as high as 3·35 kg m−2 a−1. The hydraulic state of the arroyo determines whether the alluvial valley floor is aggrading or degrading. Arroyo Chavez is incised and the alluvial valley floor is gullied and piped and is a source of sediment. The alluvial valley floor is also the portion of the basin most modified b