Anthropogenic effects along the Texas Gulf Coast - a case study of the Trinity RiverZachary A. Musselman
Continent: North America
Country: United States of America
UTM coordinates and datum: none
Climate Setting: Humid
Tectonic setting: Passive Margin
Type: Process, Stratigraphy
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The Gulf coastal plain of Texas is comprised of a dynamic coast that encompasses a multitude of depositional environments. The Texas Gulf Coast (TGC) stretches 590 km, encompasses thousands of square kilometers of estuaries and bays, and serves as the second largest tourist attraction in the state, generating seven billion dollars a year (GLO, 2002). Recently though, anthropogenic influences have been suggested as being responsible for the degradation of the TGC. Reduced sediment supply due to impoundment of coastal plain streams, coupled with relative sea-level rise, is thought to cause disruptions of geomorphic processes that help sustain wetlands (Morton, 1979; Davis, 1997). The seventh largest estuary in the United States (Pulich and White, 1991), the Galveston Bay system includes the Trinity Bay (Figure 1), which is the only natural bay-head delta in Texas that has prograded in geologically recent times (White and Tremblay, 1995).
Glacial-eustatic cycles have played a particularly influential role in sea-level effects on Texas coastal plain rivers. It appears that the reaction of coastal plain rivers to natural processes is outpaced by anthropogenic alterations to the landscape (such as impoundments and fluid withdrawal) (Morton and Purcell, 2001). In the Trinity Bay, White and Tremblay (1995) suggest that subsidence is the controlling factor of wetland loss, while recognizing upstream impoundments may also play a significant role. Previous studies of impounded streams have shown that impacts are contingent upon localized factors, and geomorphic changes downstream of dams may not be predicted without considering many variables (Friedman et al., 1998; Brandt, 2000a; Phillips, 2003a).
Numerous studies have documented the coastal plain evolution of rivers within Texas through the Holocene (Blum and Price, 1998; Rodriguez et al., 1998; Anderson and Rodriguez, 2000; Rodriguez and Anderson, 2000; Rodriguez et al., 2001; Rodriguez et al., 2000ab), with contemporary studies focusing on sedimentation rates (Longley et al., 1994; White et al., 2002), fluvial-coastal systems (Giardino et al., 1995), and sediment transport/residence time (Hudson and Mossa, 1997; Phillips, 2001; Phillips and Marion, 2001; Yeager et al., 2002; Phillips, 2003a). Recognizing that large storms often disturb coastal wetlands by causing an acceleration of routinely occurring processes (Conner et al., 1989), while simultaneously providing a mechanism for required natural processes (such as nutrient cycling), in the Trinity Bay it has been suggested that anthropogenic alterations to the landscape are often more deleterious than natural disruptions (Pulich and White, 1991).
The sediment delivery to the Trinity Bay is influenced by numerous factors including synoptic climatic patterns (location and track of a storm), the response of the river to the storm, the dam, and local geomorphic factors. The response of a river to a dam can be directly measured only if monitoring of the river occurred prior to dam construction; this is the case for the Trinity River system. The Livingston dam on the Trinity River (Figure 2) was constructed during a time when active USGS gauging stations were located above and below the impounded reach, as well as on two tributaries in the lower basin. Dams on coastal plain rivers often act as sediment traps, reducing the amount of sediment to the coast while catalyzing wetland loss. An important consideration when investigating wetland loss is the source of the sediment that is reaching the bay. Possible sediment sources in the Trinity River system include the upper basin (above the dam) and numerous sinks within the lower basin: Trinity channel, floodplain and the tributaries (Figure 3).
Case Study: Trinity River
Coastal land loss in the Trinity/Galveston Bay system has in recent years been occurring at rates between 1.5 to >3 m year-1 (shoreline retreat) with conversion of marshes to open water at a rate of 47 ha year-1 (Morton and Paine, 1990; White and Calnan, 1991; Morton, 1993; GLO, 2002). Beach erosion in much of Texas increased in the 1960s (Morton, 1977; Morton and Paine, 1990; Davis 1997) and roughly coincides with the impoundment of the Trinity and many other Texas Gulf Coast streams.
Within the lower Trinity basin, studies focusing on dam related affects have shown a notable geomorphic impact for at least 60 km downstream of Lake Livingston. Between this reach and Trinity Bay an apparent sediment "bottleneck" exists, seemingly buffering the delta/estuary system from upstream sediment regime changes (Phillips, 2003b; Phillips et al., 2004; Phillips and Slattery, 2006). The reach of river where the "bottleneck" exists is characterized by large sandy point bars, an increased occurrence of oxbow lakes and meander scars, and the channel thalweg is near or below sea-level. This fluvial-estuary transition zone has been reworked numerous times through the Holocene (Anderson and Rodriquez, 2000) and has migrated the "mouth" of the river as much as 200 km in the upstream-downstream direction (Thomas and Anderson, 1994; Phillips et al., 2004; Phillips and Slattery, 2006).
Complicating the interpretation of impoundment effects on the Trinity system, the entire post-dam period is characterized by significantly higher precipitation. Higher precipitation may have produced increased channel activity and might be masking any changes attributable to upstream coupling from the mainstem. Also contributing to increased activity in the post-dam period, three of the five largest 24-h maximum rainfall events occurred in the 1990s (Figure 4). Planform channel change in the lower Trinity River has been dynamic throughout the Quaternary. Scattered across the floodplain, oxbow lakes, meander scars and scrolls are evidence of a constantly evolving system. While the Livingston Dam has greatly reduced sediment input to the lower reaches of the Trinity River, it has not significantly altered flows. The system response is characterized by incision, widening, coarsening of channel sediment and a decrease in channel slope. The geomorphic characteristics of the lower Trinity River basin are largely dominated by Holocene sea level change and the response to extreme events, such that dam effects become relatively localized.
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