HR: 16:10h
H12F-10
Evolution of valley depth and width during base-level fluctuations

* Strong, N
stro0068@umn.edu
St. Anthony Falls Laboratory, Mississippi River at 3rd Ave. SE, Minneapolis, MN 55414 United States

Sheets, B
shee0076@tc.umn.edu
St. Anthony Falls Laboratory, Mississippi River at 3rd Ave. SE, Minneapolis, MN 55414 United States

Kelberer, M
mkelberer@attbi.com
St. Anthony Falls Laboratory, Mississippi River at 3rd Ave. SE, Minneapolis, MN 55414 United States

Kim, W
kimx0826@umn.edu
St. Anthony Falls Laboratory, Mississippi River at 3rd Ave. SE, Minneapolis, MN 55414 United States

Paola, C
cpaola@umn.edu
St. Anthony Falls Laboratory, Mississippi River at 3rd Ave. SE, Minneapolis, MN 55414 United States

We use results from an experimental study conducted in the XES subsiding-floor basin at St. Anthony Falls Laboratory to examine changes in valley width and depth during base-level fluctuations. We find that during a slow base level cycle, where the time scale of base level change was greater than the basin's theoretical equilibrium time, no clear incised valley formed, but rather a series of broad erosional surfaces. During rapid base level cycles, where the time scale of base level change was considerably less than the basin equilibrium time, incised valleys formed via a complex, step-like process: autogenic incision interacted with the externally imposed base-level change to produce multiple discrete down-cutting episodes, despite continuous base-level fall. Downstream deposition during the fall helped widen the valley downstream, producing a tapered planform comparable to that found in many natural estuaries. Deposition during the rise produced continued valley widening. Shoreline migration for both cycle frequencies was well predicted by a simple geometric model and showed no tendency to drift out of phase with base level.

http://www.safl.umn.edu/
1815 Erosion and sedimentation
1824 Geomorphology (1625)
1860 Runoff and streamflow
4556 Sea level variations
4558 Sediment transport
Hydrology [H]
2002 Fall Meeting

HR: 16:40h
H12F-12
Erosion Rate Variability in Steady State Landscapes: Sources and Implications

* Hasbargen, L
hasba002@tc.umn.edu
St. Anthony Falls Lab, and Dept. of Geology and Geophysics, University of Minnesota, 310 Pillsbury Dr. SE, Minneapolis, MN 55455 United States

Paola, C
cpaola@umn.edu
St. Anthony Falls Lab, and Dept. of Geology and Geophysics, University of Minnesota, 310 Pillsbury Dr. SE, Minneapolis, MN 55455 United States

Drainage basins experiencing long term uplift and erosion can evolve to a statistically stable form (a steady state), where the average erosion rate balances the average uplift rate. The possibility that this state develops has been demonstrated by numerical and experimental models, and by some natural drainage basins. In this paper we investigate erosion rate variability in numerical and physical experiments of drainage basin scale erosion. For numerical models, the ultimate state at steady forcing conditions is a landscape that erodes everywhere at the same rate. Prior to reaching this static steady state, numerical landscapes undergo some evolution, such that the landscape experiences adjustments that lead to lateral movement of ridges and valleys, and subsequent stabilization of the network. For small scale physical experiments, such adjustments at statistical steady state can be prolonged, and a static steady state is never reached. We focus on several mechanisms that result in local variability of erosion rate for landscapes at a statistical steady state. Such mechanisms include 1) upstream area capture; 2) hillslope failures; 3) knickpoint generation and propagation; and 4) temporary deposition in valleys. Each mechanism operates at varying time scales. Upstream area capture typically involves closure of a valley, with higher erosion rates on hillslopes in adjacent valleys, and occurs over longer time scales. Divide migration is an integral part of upstream area capture. Hillslope failures are shorter time scale processes, and involve local fluctuations in erosion rate. In our physical experiments, we have observed episodes of widespread deposition in valleys, followed by knickpoint generation and excavation of the stored sediment. Such cycles occur over the time required for a knickpoint to propagate through the network. Temporary depositional cycles in our experiment are autocyclic, that is, they arise from local process interactions. The interaction of these processes leads to fairly significant variances in erosion rate across the landscape. Characteristic patterns of erosion rate exist for ridge migration, and to a lesser degree, for temporary sediment storage and knickpoint propagation. Hillslope failure patterns do not appear to have any structure. Advances in topographic measurement, surface exposure dating, and rock exhumation rates are offering the possibility that we can measure erosion rates in field settings on a very detailed basis. Theoretical and experimental data can provide valuable insights for interpreting erosion rate variability in natural settings.

1815 Erosion and sedimentation
1824 Geomorphology (1625)
1848 Networks
Hydrology [H]
2002 Fall Meeting