HR: 1340h
AN: H53C-1276    [Abstracts]
TI: Periodic Spacing of Channel-Spanning Potholes in Navajo Sandstone, Henry Mountains Utah: Implications for Propagation of Incision Pulses across Tributary Junctions
AU: * Barnes, C M
EM: curtis\_barnes@lycos.com
AF: Department of Geosciences, San Francisco State University, 1600 Holloway Avenue, San Francisco, CA 94132 United States
AU: Sklar, L S
EM: leonard@sfsu.edu
AF: Department of Geosciences, San Francisco State University, 1600 Holloway Avenue, San Francisco, CA 94132 United States
AU: Whipple, K X
EM: kxw@mit.edu
AF: Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139 United States
AU: Johnson, J P
EM: joelj@mit.edu
AF: Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139 United States
AB: Incision of the Colorado River at Glen Canyon over the past ~1 Ma triggered pulses of incision that have migrated upstream through tributary drainages. The rate of incision of small tributaries often lags behind that of trunk streams, creatng over-steepened reaches at the tributary junction. In the arid Colorado Plateau, many of these low-order tributaries in massive lithologies have developed sequences of channel-spanning, periodically-spaced pothole bedforms. Although the dynamics of pothole formation and evolution are poorly understood, the occurrence of potholes correlates with super-critical flow and tools-limited conditions. We are exploring the hypothesis that pothole spacing and other profile attributes can be used to reconstruct the propagation of waves of incision through a drainage network. Here we report preliminary results of a field study of potholed reaches in steep, low-order channels draining Navajo Sandstone in the Henry Mountains, Utah. Our survey focused on channels where overlying sedimentary units and pediment deposits have largely been removed from the Navajo Sandstone. We surveyed six tributaries of the middle fork of Trail Canyon, and one tributary of Milk Creek, which are, in turn, minor tributaries of the Colorado River. We surveyed the long profile of each channel using a 5-m rod, 100-m tape, and a clinometer. In bedrock reaches, we focused on quantifying the spacing of alternating pothole steps and bedrock chutes, and the elevation change within and between potholes steps. We also characterized individual potholes by width, depth, maximum sediment size captured, and direction of flow recirculation. In alluviated reaches, we measured the bankfull width and depth to estimate relative differences in bankfull discharge. We also measured channel geometry in higher-order trunk streams at the junctions with the surveyed tributaries. Our preliminary results include the successful prediction of locations where potholed channels were found, by comparing topographic and geologic maps to estimate the character and quantity of coarse sediment supply. Where diorite pediments and overlying sedimentary rocks have been removed by erosion from above the Navajo Sandstone, pothole channels are common. Although the presence of grinders contributes to pothole abrasion, observations suggest that low supply rates of coarse sediment are essential for pothole development. Particularly important is low supply of the relatively hard diorite derived from the laccolithic cores of nearby Mt. Hillers and Mt. Holmes. In many potholes the only sediments present were sand-sized and finer. In each of the surveyed tributary channels we observed regular spacing of pothole bed forms. Preliminary analysis suggests a characteristic spacing of pothole steps that varies systematically with reach gradient, drainage area and other tributary attributes. Further analysis will allow us to explore the relationships between the morphology of potholed channels and the relative incision rates of the tributary and the trunk streams. It may be possible, for example, to assess the relative incisional efficiency of tributaries with differing extents of pothole development.
DE: 1815 Erosion and sedimentation
DE: 1824 Geomorphology (1625)
DE: 1848 Networks
DE: 1869 Stochastic processes
DE: 1886 Weathering (1625)
SC: Hydrology [H]
MN: 2004 AGU Fall Meeting