Anderson 9417798 Coupled models of topographic evolution that include both geomorphic and tectonic processes are becoming more common as a tool to understand the rich variety of feedbacks within the system, as illustrated by the September 1992 Chapman Conference on Tectonics and Topography. Tectonic processes accomplish differential uplift of rock, creating discrete mountain cores whose geometries are set by either discrete faults or by distributed deformation. Channels incised into this rock mass bound local hillslopes which transport weathering-produced particles, or which fail in landslides. The channels then convey this material out of the mountain front, where it accumulates in local basins or is transported in alluvial valleys to the ocean. The mountain mass is thereby lightened by the degree to which relief is generated, while the range-bounding basins are loaded. The resulting flexure associated with isostatic response is therefore driven by the geomorphology and is intimately tied to the ability of rivers to incise into bedrock. In addition, the local relief sets the pattern of topographically-induced stresses in the valley, driving the pattern of microcracking, with the potential feedback of enhancing local incision rates (Miller, 1993). We will study bedrock channel evolution through a set of related efforts including field, experimental, and theoretical work. While our goal is to develop a quantitative, process-based understanding of the incision of bedrock by rivers, we are motivated strongly by the problem of landscape evolution at a scale relevant to tectonics. To accomplish this we will strive toward establishment of a physically-based channel incision rule that will be faithful to the detailed morphology of a bedrock channel and to the details of the hydrological forcing of such systems, and useful to the community attempting to construct coupled landscape evolution models. We feel strongly that the present models capture only broadly the incision process, and are limited in their ability to trigger the important feedbacks and to adequately assess the roles of environmental variables of interest: climate, lithology, tectonics. The strongly nonlinearity of the incision process should emphasize the roles of large hydrologic events, either meteorological or catastrophic (ice or landslide dambursts) in origin. The degree of nonlinearity in the process response to a hydrologic forcing is therefore crucial to understand. As an extreme example, if the nonlinearity is so strong that only the damburst floods cause erosion, the local channel incision becomes intimately coupled to mass wasting processes upstream, and less coupled to meteorological events, making the coupling internal to the geomorphic system.
