Toledo Bend reservoir and geomorphic response in the lower Sabine River

Abstract

Downstream geomorphic responses of stream channels to dams are complex, variable, and difficult to predict, apparently because the effects of local geological, hydrological, and operational details confound and complicate efforts to apply models and generalizations to individual streams. This sort of complex geomorphic response characterizes the Sabine River, along the Texas and Louisiana border, downstream of the Toledo Bend dam and reservoir. Toledo Bend controls the flow of water and essentially prevents the flux of sediment from three-quarters of the drainage basin to the lower Sabine River. Although the channel is scoured immediately downstream of the dam, further downstream there is little evidence of major changes in sediment transport or deposition, sand supply, or channel morphology attributable to the impoundment. Channels are actively shifting, banks are eroding, and sandbars are migrating, but not in any discernibly different way than before the dam was constructed. The Sabine River continues to transport sand downstream, and alluvial floodplains continue to accrete. The relatively small geomorphic response can be attributed to several factors. While dam releases are unnaturally flashy and abrupt on a day-to-day basis, the long-term pattern of releases combined with some downstream smoothing creates a flow regime in the lower basin which mimics the pre-dam regime, at least at monthly and annual time scales. Sediment production within the lower Sabine basin is sufficient to satisfy the river's sediment transport capacity and maintain pre-dam alluvial sedimentation regimes. Toledo Bend reservoir has a capacity: annual inflow ratio of 1.2 and impounds 74% of the Sabine drainage basin, yet there has been minimal geomorphic response in the lower river, which may seem counterintuitive. However, the complex linked geomorphic processes of discharge, sediment transport and loads, tributary inputs, and channel erosion include interactions which might increase as well as decrease sediment loads. Furthermore, if a stream is transport-limited before impoundment, the reduced sediment supply after damming may have limited impact.

Avulsion regimes in southeast Texas rivers

“…Mainstem reservoirs on the other study rivers are for water supply or hydroelectric power, and do not appear to have changed the general flow regime (Hudson and Mossa, 1997;Phillips, 2003;Wellmeyer et al, 2005). Sam Rayburn, however, is a flood control impoundment.…”

Channel adjustments of the lower Trinity River, Texas, downstream of Livingston Dam

“…Thus the lower river is an actively migrating channel and has been throughout the Quaternary. The alluvial morphology and stratigraphy of the lower Trinity (and the nearby and similar Sabine River), and the deposits and paleochannels now submerged in Trinity and Galveston Bays and the Gulf of Mexico preserve evidence of climate, sea level and upstream sediment delivery changes Anderson et al, 1992;Blum et al, 1995;Phillips, 2003;Phillips and Musselman, 2003;Rodriguez and Anderson, 2000;Rodriguez et al, 2001;Thomas and Anderson, 1994). Thus one task is to disentangle dam effects from other factors influencing the geomorphology of the lower Trinity.…”

Channel change at Toudaoguai Station and its responses to the operation of upstream reservoirs in the upper Yellow River

“…Associated with the time of reservoir operation and the consistency of physical environmental factors, it is clear that the annual variations of water and sediment at Toudaoguai cross-section are triggered by upstream reservoir operation. When incoming sediment is trapped inside the reservoirs, the released sediment-free water has relatively strong energy in eroding downstream channel bed and banks, just as discussed earlier, channel bed armoring will occur at first in the channel immediately downstream of the dams and then extend downstream gradually (Stanley, 1996;Phillips, 2003;Zahar et al, 2008), the relatively smaller sand in particle size is entrained and transported downstream to reach a dynamic equilibrium between sediment load supplied to the stream and flow's sediment carrying capacity. The phenomenon of channel bed armoring becomes increasing unapparent as the distance to upstream dams increases, and finally disappears.…”

“…However, the construction and operation of these hydraulic engineering works will typically modify the river stream hydrology and sediment dynamics for the fluvial systems downstream from dams, leading river channels downstream of dams to adjust accordingly so that they could adapt to the altered input conditions (Schumm, 1969;Gregory, 1977;Hooke, 1997;Phillips, 2003). With regard to the channel adjustment and other related issues, previously numerous research works have been carried out and some constructive conclusions were made.…”