IGPP Seminar Series

What controls landscape scale? Connecting models and morphology

by Dr. Taylor Perron
UC Berkeley

Abstract

Landscapes often exhibit characteristic spatial scales. Two widespread examples are the finite extent of landscape dissection by branching streams, and the periodic spacing of adjacent valleys. I use a numerical landscape evolution model to show that both of these characteristic scales emerge from a competition between advective (stream incision) and diffusive (soil creep) erosion processes. The model solutions reveal that (1) landscape dissection by branching streams stops at a spatial scale for which the ratio of advective to diffusive timescales is approximately unity, and (2) valley spacing varies linearly with this ratio: as stream incision becomes more effective relative to erosion by soil creep, the spacing between adjacent valleys narrows. I present a field test of this scaling relationship by comparing the strongly periodic topography (~170m spacing between valleys) of the Gabilan Mesa, a landscape in central California, with the model prediction. The topography of the Gabilan Mesa has developed through the erosional dissection of a formerly planar alluvial surface, of which some portions are preserved. Surface exposure and burial ages derived from concentrations of cosmogenic 10Be and 26Al in quartz bracket the time when erosional dissection of this surface began. Dividing this timescale into the elevation of the initial surface above the present ground surface gives a time-averaged erosion rate for the landscape. Using this erosion rate and topographic profiles of hillslopes and stream channels, I solve for the parameters in the diffusive and advective erosion terms. I then calculate the ratio of advective to diffusive timescales for the Gabilan Mesa and show that the valley spacing predicted by the scaling relationship compares well with the observed valley spacing.
Tuesday, 18 April 2006
3845 Slichter Hall
Refreshments at 3:45 PM
Lecture at 4:00 PM