Paper No. 9
Presentation Time: 10:30 AM
THE TOPOGRAPHIC IMPRINT OF SPATIAL AND TEMPORAL PATTERNS OF ROCK UPLIFT ENCODED IN BEDROCK RIVER PROFILES
At catchment scale, the relationship between landscape form and rock uplift relative to baselevel is dictated by the response of rivers. Consequently, stream profile analysis has proven a powerful tool for extracting quantitative information about both spatial and temporal patterns in the rate of rock uplift relative to baselevel. Landscapes of the western US that have played an important role in our growing understanding of these relationships are used as illustrative examples. Results show that catchment-mean normalized channel steepness index (ksn) increases monotonically with erosion rate (E) or rock uplift rate (U). There is, however, no universal relationship between ksn and E or U. The channel steepness required to drive erosion at a rate sufficient to balance the relative rock uplift varies significantly as a function of both substrate properties and climate. The relationship with climate appears to be complex, and depends on at least: (1) thresholds of motion and detachment; (2) the magnitude and frequency of floods; and (3) the sediment load. Interestingly, whereas in dry regions, erosion rate for a given channel steepness is always predicted to increase rapidly with mean annual rainfall, in humid settings this is not always the case. Until this relationship, and that between substrate properties and erosional efficiency, is quantified, stream profile analysis will be limited to diagnosing relative patterns in rock uplift rate unless locally calibrated. Also, spatial patterns in substrate properties, spatial patterns in rock uplift rate, and temporal changes in rock uplift rate or climate can produce similar landforms. Fortunately there are measurable diagnostic criteria that can be used to determine the driving factors. Finally, rivers respond to changes in slope, so the spatial scale of rock uplift relative to catchment size influences landscape response. Although regional mantle-driven rock uplift will have little effect on small- to moderate-sized catchments, even small tilts can significantly change the slope of large rivers, and thus can trigger incision into wedges of sediment or drainage re-organization that can involve significant baselevel fall. When coupled with isostatic rebound, these effects can induce a large geomorphologic response to a small perturbation to the regional slope.