Sediment Retention in Stream Corridors of the Lower Trinity River Basin, Texas

Study obiectives This study has the following objectives: 1) Characterize the historic discharge and sediment data at stations on the lower Trinity River. 2) Assess changes in channel and floodplain morphology, alluvial sedimentation, and sediment delivery at the river mouth following completion of the dam. 3) Evaluate the sediment inputs from upland erosion, tributaries, and bank erosion downstream of Lake Livingston. . 4) Characterize the channel and floodplain sediment above and below the lake and dam site. 5) Identify the difference in sediment transport to the upper delta before and after the dam placement. 6) Determine the sediment sources that could replenish the stream sediment supply that are near the river channel or in the lake. It should be understood that the first year of this study will allow preliminary, tentative or speculative conclusions on the six objectives above. StudY Desian and Methods The general strategy is based on assessment of channel and floodplain changes on the Trinity River between Livingston Dam and Trinity Bay. The assessments are based on field mapping, dendrogeomorphology and vegetation evidence, sediment sampling, field observation of flood impacts, and analysis of maps, aerial photographs, digital orthophotoquads, and stream gage records. The general approach is based on the notion that downstream effects of the dam on flow and sediment regimes will be integrated and reflected by changes in channel morphology. For example, channel incision should be evidenced by steep banks, exposed tree roots, and other morphological indicators such as erosion scarps, undercuts, or slope failures. This will be supplemented by examination of channel and floodplain sediments upstream and downstream of the lake. Sediment analyses will focus on low-cost methods to determine or estimate sediment sources and residence times, such as color, redox status, grain angularity, and grain coatings of iron oxides. Similar methods have previously been successfully used in U.S. Forest Service studies in east Texas. Field mapping will be conducted with standard in conjunction with digital orthophotoquads at 2.5 m resolution stored and displayed in the field on a laptop computer. Vegetation is an important indicator of channel change. Exposed roots, for example, indicate erosion and stripping, while adventitious roots are useful in dating burial episodes. The burial of basal flares and root crowns indicates deposition, and can be used to date and estimate rates of accretion. These dendrogeomorphic methods are described in detail by C.R. Hupp and colleagues (Hupp and Bazemore 1993; Hupp and Morris 1990; Huppand Osterkamp 1996; Hupp and Simon 1991). Dendrogeomorphic methods were successfully used on Loco Bayou in east Texas by Phillips (2001; Phillips and Marion 2001).