CHARACTER OF HEADWATERS ADJUSTMENT TO BASE-LEVEL DROP, INVESTIGATED BY DIGITAL MODELING

Using a computer model written for dynamic river profile modeling (Snow and Slingerland, 1987), long-term profile adjustments to constant-rate base level lowering/tectonic block uplift have been simulated with varied conditions of out-flow discharge (10 and 100 m3/s), sediment diameter (0.1, 0.4 and 2.0 mm) and sediment load (concentrations of 1200 and 400 ppm by weight). Twelve sets of initially graded profile data cover all possible combinations of the variables. Each modeled stream has been subjected to each of three base level drop rates (0.1, 1.0 and 10.0 mm/yr), for 10,000 years of model time. Modeled histories of headwaters erosional response differ in two ways. The first is length of lag time of response, characterized by a period of quiescence in erosion at the headwaters prior to the channel steepening sufficiently for erosion to commence at this upstream point. Variations in discharge and sediment concentration affect the lag time of response in all cases. The second variable aspect of response is the relative magnitude of eventual headwaters erosion. The main control is the drop rate; a high drop rate yields a low relative magnitude whereas a low drop rate yields a high relative magnitude of response. The rate of base level drop also controls to what degree the other controlling variables of discharge, sediment concentration, and sediment diameter affect the eventual magnitude of headwaters erosion. Rates of trunk stream and headwaters erosion indicate that 21 out of 36 of the modeled stream systems attain a condition approximating parallel profile erosion after 10,000 years. Some examples, notably those with low power to erode and high imposed drop rates, exhibit headwaters erosion rates that continue to be well below rates of base level lowering, creating profile steepening with no apparent time limit.