Historical Channel Adjustment and Estimates of Selected Hydraulic Values in the Lower Sabine River and Lower Brazos River Basins, Texas and Louisiana
The U.S. Geological Survey, in cooperation with the Texas Water Development Board, evaluated historical channel adjustment and estimated selected hydraulic values at U.S. Geological Survey streamflow-gaging stations in the lower Sabine River Basin in Texas and Louisiana and lower Brazos River Basin in Texas to support geomorphic assessments of the Texas Instream Flow Program. Channel attributes including cross-section geometry, slope, and planform change were evaluated to learn how each river's morphology changed over the years in response to natural and anthropogenic disturbances. Historical and contemporary cross-sectional channel geometries at several gaging stations on each river were compared, planform changes were assessed, and hydraulic values were estimated including mean flow velocity, bed shear stress, Froude numbers, and hydraulic depth. The primary sources of historical channel morphology information were U.S. Geological Survey hard-copy discharge-measurement field notes. Additional analyses were done using computations of selected flow hydraulics, comparisons of historical and contemporary aerial photographs, comparisons of historical and contemporary ground photographs, evaluations of how frequently stage-discharge rating curves were updated, reviews of stage-discharge relations for field measurements, and considerations of bridge and reservoir construction activities. Based on historical cross sections at three gaging stations downstream from Toledo Bend Reservoir, the lower Sabine River is relatively stable, but is subject to substantial temporary scour-and-fill processes during floods. Exceptions to this characterization of relative stability include an episode of channel aggradation at the Sabine River near Bon Wier, Texas, during the 1930s, and about 2 to 3 feet of channel incision at the Sabine River near Burkeville, Texas, since the late 1950s. The Brazos River, at gaging stations downstream from Waco, Texas, has adjusted to a combination of hydrologic, sedimentary, and anthropogenic controls. Since the 1960s, numerous point bars have vertically accreted and vegetation has encroached along the channel margins, which probably promotes channel-bed incision to compensate for a reduction in cross-sectional area. Channel incision was detected at all gaging stations along the Brazos River, and the depth of incision is greatest in the lowermost gaging stations, exemplified by about 5 feet of channel-bed incision between 1993 and 2004 at Richmond, Texas. One notable exception to this pattern of incision was a period of aggradation at U.S. Geological Survey gaging station 08096500 Brazos River at Waco, Texas, during the late 1920s and 1930s, probably associated with upstream dam construction. Lateral channel migration rates along the Brazos River determined from aerial photographs are greatest between Waco and Hempstead, Texas, with numerous bends moving an average of more than 10 feet per year. Migration rates at selected bends downstream from Hempstead were measured as less than 10 feet per year, on average. Two tributaries of the Brazos River, the Little and Navasota Rivers, also were investigated for historical channel adjustment. The Little River near Cameron, Texas (08106500) has incised its channel bed about 12 feet since 1949, and the lower Navasota River shows complex adjustment to bridge construction activities and a channel avulsion.