Print PageThere is a need to understand how potential flow modifications from dams may impact the health of riparian vegetation communities. As part of The Texas Instream Flow Program, our study seeks to know if Toledo Bend Dam on the Sabine River along the Texas/Louisiana border has (1) altered flooding downstream, and if so, (2) whether conditions of bottomland hardwood forests are in decline. River discharge data for two USGS gauging stations downstream of the dam was analyzed by aggregating all flood events into 5-year periods to evaluate trends in flood frequency, duration, and peak and total discharge in those periods before (1926−1965) and after dam construction (1971−2005). Riparian habitat was characterized by establishing replicate 100-m2 plots in a stratified random approach within three topographic zones (3 plots × 3 zones: levee, mid-floodplain, slough, for a total of 9 plots per site) at three selected sites below the dam. Key riparian vegetative indicators included: age class distribution, richness and diversity, density, and % canopy cover. In each plot, we surveyed all trees >3 cm diameter, surveyed shrubs < 3 cm DBH using line intercept methodology, surveyed herbaceous vegetation using a line point intercept methodology, and collected soil samples to determine soil pH. Each species was classified as Obligate Wetland−OBL, Facultative Wetland−FACW, Facultative−FAC, and Facultative Upland−FACU.
Results/Conclusions
Total discharge (p<0.01) and flood duration (p<0.01) were reduced, and flood frequency increased (p=0.04) post-dam construction within 90 km downstream of Toledo Bend dam; however, peak discharge was no different. By 180 km downstream of the dam, there was no difference pre- and post-dam construction among any of the four variables (peak discharge, total discharge, duration, frequency). Historical vegetation data were unavailable for the Sabine River floodplain; thus, results from this study convey vegetation composition at the present time. We found that flood-dependent species were sensitive to elevational changes of 0.7 m. OBL species of trees, shrubs, and herbs were largely restricted from the levee and mid-floodplain areas, which had the highest soil pH (5.75 ± 0.05 and 5.30 ± 0.05), while FACU and FAC species were mostly excluded from slough areas, which had the lowest soil pH (4.93 ± 0.04).FACW species had similar occurrences among all topographic plots. These results suggest that minor changes in flooding elevations might limit occurrence of wetland species to the lowest topographic zones. Furthermore, OBL trees had the greatest diameters in size-class distribution, suggesting that recruitment has been limited.
