New Mexico Geological Society Annual Spring Meeting — Abstracts

While it has been proposed that the rate of vertical fluvial incision of a stream is proportional to stream power, no field-based tests have been conducted to validate this proposed relationship. The general form of this relationship is dz/dt=kQ^mSⁿ, where dz/dt is the vertical incision rate, k is a basin-specific constant, Q is discharge, .S is the energy slope, and m and n are coefficients that scale the relative influence of discharge and slope. At least 3 different combinations of m and n values have been suggested as most appropriate for predicting rates of vertical fluvial incision and all previous stream power law tests have substituted drainage basin area for discharge. Using a field-based approach, we determined discharge-drainage area relationships and stream power values for the Red River and Rio Hondo in northern New Mexico. These two streams both head in the Taos Range, and flow west onto the Taos plateau. Previous workers have determined approximate rates of vertical incision that allow for a field-based test of the stream power law. Field measurements of discharge for the two streams show that discharge generally increases with drainage area for these two basins, but discharge does decrease in the more alluvial stretches (developed in basin fill) downstream of the mountain front. Discharge then increases once the streams enter lower gorges incised through basalts. We suggest that the stream loses water to the relatively permeable basin fill sections, while it gains water once it intersects a regional groundwater system in the basalts of the lower gorges. Stream power maps using field-derived data indicate that unit stream power (m=.5, n=1) and another variant (m=n=1) best predict the rates of incision for both the Red River and Rio Hondo. Furthermore, field-based stream power maps match incision rates better than their map-based counterparts. Similarities between the map-based and field-based unit stream power distributions suggest that this may be the best form for predicting rates of vertical incision using just maps. We suggest that the similar, generally parallel profile shapes of both the modem valleys and terrace treads for both rivers (indicating constant rates of incision spatially) suggest that these streams are at grade. If so, the stream power law parameterized for unit stream power (m=0.5, n=1) provides a potential simple test for grade of river systems in areas where base level fall or regional tectonics cause net fluvial incision.