FLUVIAL INCISION IN THE UPPER NUECES RIVER BASIN, CENTRAL TEXAS

The stream power incision model, in which drainage area serves as a proxy for discharge, is a popular method of quantifying fluvial incision. This study examines how well drainage area serves as a proxy for discharge in the semi-arid climate of central Texas, where precipitation is characterized by infrequent, high intensity, localized storms. Fluvial incision rates are calculated with the stream power incision model using drainage area, mean peak annual discharge, and 1.0-, 1.5-, and 4.0-year-recurrence-intervals which bracket bankfull discharges at fourteen points in eight detachment-limited, mixed bedrock-alluvial streams underlain by marls and limestones. This study utilizes 30m resolution USGS DEM analysis and USGS gauging station data for the W. Nueces, Nueces, Dry Frio, Frio, Sabinal, and Medina Rivers and Seco and Hondo Creeks. A range of predetermined values for erosional coefficient K and exponents m and n in the model are from published values of K for similar lithologies (mudstones, sandstones) and the relation (m/n) is ~0.5, with m and n values usually ranging between 0.3-0.5 and 0.6-1.0 respectively. Calculated incision rates are compared to each other and to independently constrained incision rates from fluvial terrace incision observed in six streams in the field using DGPS surveys (Nueces, Frio, Sabinal, and Medina Rivers; Seco and Hondo Creeks). We find that: (1) the range of incision rates calculated with the stream power incision model using mean peak annual discharge and 1.5- and 4.0-year-recurrence-interval discharges falls within the range of incision rates obtained from field data (4x10^-6 - 5x10^-3 m/yr); and (2) drainage area values most closely approximate mean peak annual discharge and 4.0-year-recurrence-interval discharge values in the stream power incision model, though often significantly overestimate them. Thus, using drainage area as a proxy for discharge in the stream power incision model in semi-arid climates on geologically similar passive margins is appropriate where historical discharge data is unavailable.