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EROSION OF THE RIO PUERCO BASIN, NEW MEXICO – FIRST COSMOGENIC ANALYSIS OF SEDIMENTS FROM THE DRAINAGE NETWORK OF A LARGE WATERSHED
We determined that the Rio Puerco Basin is lowering about 100 m My-1 by measuring 10Be and 26Al in channel alluvium quartz from 37 sites distributed among drainage areas ranging from 170 km2 to 16000 km2. The combination of easily eroded lithologies, sparse vegetation, and monsoon-dominated rainfall result in rapid sediment generation and efficient sediment delivery from tributaries to the main Puerco channel.
Measured activities of 10Be range from 0.5 to 24 * 105 atom g-1 and can be interpreted as rates of sediment generation that range from 2.1*104 kg km2 y-1 to 1.0 *106 kg km2 y-1, the equivalent of rock erosion at 8 to 400 m My-1 (
r=2600 kg m3). Basin to basin variance is high for smaller basins (<2000 km2) but dampens at larger scales. The weighted average rock erosion rate for 16 headwater basins (m=440 km2) is 100 m My-1, similar to that (103 m My-1) calculated from 10Be activity of sediment collected just below the confluence of the Rio Puerco’s two largest sub-basins (14200 km2). Our nuclide-based denudation estimates are consistent with previously published values determined by several different methods.Three results suggest an underlying dynamic equilibrium of upland sediment production and delivery of alluvium to the Rio Puerco over the Holocene. 1) 10Be-based sediment generation rates are well correlated with bedrock lithology. 2) 20th century sediment yields for 5 USGS gages are similar to 10Be-based estimates for the same drainage basins. 3) Fifteen samples from a 7 m vertical arroyo wall deposited over several thousand years, but exposed by channel incision in the late 19th century, have similar 10Be activities (1.08 +/-0.10 * 105 atom g-1) demonstrating that nuclide abundance in sediment does not change significantly over time.
This study demonstrates cosmogenic nuclides have the potential to provide rapid assessment of long-term erosion rates not only in small, homogeneous basins but over a wide range of tributary areas, lithologies, and vegetation covers.