MODERN COMPARED TO GEOLOGIC RATES OF EROSION IN ARROYO CHAVEZ, RIO PUERCO BASIN, NEW MEXICO

The need to understand how human activities effect natural rates of erosion is essential in defining degraded landscapes. This study examined and compared erosion rate estimates made in Arroyo Chavez (2.28 km²), a subbasin of the Rio Puerco, New Mex., using techniques at two different time scales, the modern and geologic. Between 6/13/1996 - 10/02/1998, 29 to 58 rainfall/runoff events were sampled with sediment traps and small dams. The spatial scale of measurements ranged from 0.76 to 2,280 m². Average erosion rates measured for five geomorphic elements were: gently sloping hillslopes 0.17 mm/yr; mesa 0.29 mm/yr; steep colluvial slopes 0.46 mm/yr; alluvial fan 0.74 mm/yr; and the alluvial valley floor 1.69 mm/yr. Basinwide erosion measured at the outlet of the basin using suspended-sediment data show erosion of 0.78 mm/yr over the 2-years. At the geologic time scale (>10⁴ years) sediment production rates for Arroyo Chavez were measured on the geomorphic elements using in situ produced cosmogenic radionuclide 10Be. The 10Be data demonstrate effective fluvial integration where the average 10Be activity measured in the stream channel sediment (1.57 ± 0.18 x 10⁵ atoms g-1) is statistically indistinguishable from the 10Be activity measured in the geomorphic elements (0.87 to 1.89 x 10⁵ atoms g-1). This similarity indicates a dynamic equilibrium of sediment transport over geologic time; thus, channel sediment nuclide concentrations are representative of basin sediment nuclide concentrations. The basinwide erosion measured with 10Be is 10² ± 24 m/Myr (0.1 mm/yr). This geologic rate of erosion plots near the range of values of modern erosion rates (0.11 to 2.93 mm/yr) and indicates that these two independent methods (modern vs. isotopes) are both valid measurement techniques.

The difference in erosion rates between the two methods was most noticeable for the alluvial valley floor, where the modern rate of erosion was as high as 2.93 mm/yr. The alluvial valley floor is grazed and has gas pipeline activity. Can this difference in the geologic scale basinwide erosion rate of 0.1 mm/yr be contrasted to the modern erosion rate on the alluvial valley floor and be used to say that human influences have caused an order of magnitude increase on the alluvial valley in Arroyo Chavez?