EVOLUTION OF OXBOW LAKES ALONG THE BRAZOS RIVER, TEXAS

Oxbow lakes are a major component of the fluvial system in lowland regions. After cutoff, the lake becomes filled with sediment and requires an increasing discharge from the main channel to maintain a hydrologic connection. This connection is necessary for diverse aquatic habitat and to provide storage for sediment. The relationship between the cutoff ratio, diversion angle and sedimentation rate of twenty eight lakes located on the middle and lower Brazos River, central Texas, were examined. The time of cutoff was dated using historic maps and aerial photographs. The sediment within the Brazos River system is largely fine sands, silt and clays. The sedimentation rate was determined using a digital elevation model to determine the thickness of sediment in the oxbow relative to the main channel. Average rates of sedimentation range from 0.02 to 0.4 meters per year. Field observations of main channel-to-lake connections for five of the lakes were compared to their rates of sedimentation to determine a correlation between discharge events in the main channel and the sediment deposited in the lake. The rates of sedimentation increase as the area of the lake increases. In addition, oxbows with a larger cutoff ratio have a greater number of connections, suggesting sediment is deposited over a wider surface area during flooding. These findings compare well with studies of oxbows occurring in varied environments with different sediment regimes.