HR: 17:45h
AN: H54B-08    [Abstracts]
TI: Relating Channel-Geometry Changes to Stream Discharge and Sediment Discharge at the Pine Creek Bridge, Upper Yellowstone River, Montana
AU: * Holnbeck, S R
EM: holnbeck@usgs.gov
AF: U.S. Geological Survey, 3162 Bozeman Avenue, Helena, MT 59601
AB: Selected channel-geometry, stream-discharge, and sediment-discharge data were collected by the U.S. Geological Survey in cooperation with the Park Conservation District and the Montana Department of Transportation, from 1999 through 2001 at the Pine Creek Bridge on the upper Yellowstone River, Montana to provide information for various studies following large floods (100-year recurrence interval) in 1996 and 1997. Upstream from the bridge, the upper Yellowstone River is a high-gradient (about 14 feet per mile) coarse-bed stream and has a drainage area of about 3,500 square miles. Runoff results primarily from snowmelt. The Pine Creek Bridge spans the entire width (about 270 feet) of the Yellowstone River and, except for localized scour effects at two piers, the cross section at the bridge is similar in geometry to natural cross sections located upstream and downstream. Channel-geometry was measured 11 times along the upstream side of Pine Creek Bridge during various stream-discharge conditions that ranged from 25,100 cubic feet per second (about 140 percent of bankfull discharge) during spring runoff to a low-flow summer discharge of 2,220 cubic feet per second. Bedload and suspended-sediment concentration data also were collected and were used to estimate bedload discharge, suspended-sediment discharge, and total-sediment discharge changes. These data were used to relate channel-geometry changes at the bridge cross section to changes in stream discharge and sediment discharge during the 1999 through 2001 period. Channel-geometry changes (excluding local scour at the piers) were quantified by algebraically summing differences in streambed elevation along the cross section from one measurement time to the next. The algebraic differences in streambed elevations were used to define streambed degradation and aggradation depths, which were summarized statistically and compared to stream discharge and sediment discharge through the range of the data. The largest mean and maximum degradation depths (1.58 feet and 3.19 feet, respectively) were determined at the largest measured discharge (25,100 cubic feet per second) and the lowest mean and maximum degradation depths (0.13 feet and 0.43 feet, respectively) were determined at a relatively low measured discharge (4,650 cubic feet per second). The largest mean and maximum aggradation depths (0.75 feet and 2.39 feet, respectively) were determined at a measured discharge of 15,600 cubic feet per second. The smallest mean and maximum aggradation depth (0.22 feet and 0.99 feet, respectively) were determined at the lowest measured discharge (2,220 cubic feet per second). Variations in aggradation depth with discharge generally were small through most of the range of measured stream discharges. Linear regression equations relating logarithms of mean streambed degradation depth to logarithms of stream discharge and sediment discharge showed reasonable linear correlation (r$^{2}$ = 0.76 and 0.74, respectively), as did equations relating logarithms of maximum streambed degradation depth to logarithms of stream discharge and sediment discharge (r$^{2}$ = 0.70 and 0.64, respectively). Linear regression equations relating logarithms of mean streambed aggradation depth to logarithms of stream discharge and sediment discharge were less strongly correlated (r$^{2}$ = 0.57 and 0.54, respectively), and equations relating logarithms of maximum streambed aggradation depth to logarithms of stream discharge and sediment discharge showed no correlation (r$^{2}$ $<$ 0.01).
DE: 1815 Erosion and sedimentation
SC: Hydrology [H]
MN: 2004 AGU Fall Meeting