HR: 17:45h
AN: H34A-08    [Abstracts]
TI: Longitudinal Stream Profile Morphology and Patterns of Knickpoint Propagation in the Bighorn Range
AU: * Safran, E B
EM: safran@lclark.edu
AF: Environmental Studies Program, Lewis and Clark College, 0615 SW Palatine Hill Road, Portland, OR 97219 United States
AU: Anderson, R S
EM: andersrs@buffmail.colorado.edu
AF: Department of Geological Sciences and INSTAAR, University of Colorado, Boulder, 1560 30th Street, Boulder, CO 80304 United States
AU: Riihimaki, C A
EM: criihima@brynmawr.edu
AF: Department of Geology, Bryn Mawr College, 101 N. Merion Avenue, Bryn Mawr, PA 19010 United States
AU: Armstrong, J
EM: jonny5armstrong@gmail.com
AF: Biology Department, Lewis and Clark College, 0615 SW Palatine Hill Road, Portland, OR 97219 United States
AB: The northern U. S. Rocky Mountains and the adjacent sedimentary basins are in a transient state of response to regional, Late Cenozoic exhumation. Assembling the history of landscape change there requires interpreting the morphology and genesis of transient landforms such as knickpoints in longitudinal stream profiles. We used concavity and normalized channel steepness indices to quantify the longitudinal profile morphology of >75 streams draining the east side of the Bighorn Range and the adjacent Powder River Basin. Our analyses show that individual units in the range-margin sedimentary cover rock exert a strong influence on longitudinal profile morphology. In the Tongue River and Crazy Woman Creek drainages, more than 50% of the streams examined had knickpoints localized within a resistant, siliceous dolomite. Knickpoints on most streams with drainage areas greater than ~100 km² at the range front have migrated headward into the gneissic and plutonic core of the range. In the Clear Creek drainage, where the lateral extent of sedimentary cover rock is more restricted than in the adjacent drainages, knickpoints do not align with any particular unit. River profiles in the Powder River Basin beyond 10-20 km from the range front exhibit concavities of ~0.3-0.6 and normalized channel steepness indices of 40-60 (using 0.45 as a reference concavity). All profiles analyzed that extend into the mountain range exhibit at least one reach with exceptionally high (>2) concavity and relatively high (100-600) normalized channel steepness index, highlighting zones of transient adjustment to local base-level drop in the basin. Headwater reaches of range-draining streams exhibit variable but moderate values of concavity (0.15-0.9) and normalized channel steepness index (20-100). The varied morphology of these reaches reflects their passage across a relict surface of low relief but also the effects of glaciation and/or the signature of the narrow summit spine that caps the range.
DE: 1815 Erosion
DE: 1819 Geographic Information Systems (GIS)
DE: 1824 Geomorphology: general (1625)
DE: 1825 Geomorphology: fluvial (1625)
DE: 1856 River channels (0483, 0744)
SC: Hydrology [H]
MN: Fall Meeting 2005