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Anthropogenic effects along the Texas Gulf Coast - a case study of the Trinity River

Zachary A. Musselman
Millsaps College

Location

Continent: North America
Country: United States of America
State/Province: Texas
UTM coordinates and datum: none

Setting

Climate Setting: Humid
Tectonic setting: Passive Margin
Type: Process, Stratigraphy


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Trinity River Basin


Livngston Dam


Sediment source below the dam


24-hour rainfall maxima


Description

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.

Associated References:

  • Anderson, J.B., Rodriquez, A.B., 2000. Contrasting styles of sediment delivery to the east Texas shelf and slope during the last glacial-eustatic cycle: implications for shelf-upper slope reservoir formation. Gulf Coast Assoc. Geological Societies Transactions 50, 343- 347.
  • Blum, M.D., Price, D.M., 1998. Quaternary alluvial plain construction in response to glacio-eustatic and climatic controls, Texas Gulf Coastal Plain. Relative Role of Eustasy, Climate, and Tectonism in Continental Rocks. SEPM Special Publication 59, 31-48.
  • Brandt, S.A., 2000a. Classification of geomorphological effects downstream of dams. Catena 40, 375-401.
  • Conner, W.H., Day, J.W.Jr., Baumann, R.H., Randall, J.M., 1989. Influence of hurricanes on coastal ecosystems along the northern Gulf of Mexico. Wetlands Ecology and Management 1 (1), 45-56.
  • Davis, Jr., R.A., 1997. Regional coastal morphodynamics along the United States Gulf of Mexico. Journal of Coastal Research 13 (3), 595-604.
  • Friedman, J.M., Osterkamp, W.R., Scott, M.L., Auble, G.T., 1998. Downstream effects of dams on channel geometry and bottomland vegetation: regional patterns in the Great Plains. Wetlands 18, 619-633.
  • Giardino, J.R., Lynch, K.M., Jennings, D.O., 1995. Coupling the fluvial-coastal system with rising sea level: the Texas Gulf Coast paradigm. In Norwine, J., Giardino, J.R., North, G.R., Valdes, J., (Eds), The Changing Climate of Texas. Geobooks, College Station, Texas, 300-321.
  • GLO (Texas General Land Office), 2002. Coastal Issues. http://www.glo.state.tx.us/coastal.html, accessed 10/26/05.
  • Hudson, P.F., Mossa, J., 1997. Suspended sediment transport effectiveness of three large impounded rivers, U.S. gulf coastal plain. Environmental Geology 32 (4), 263-273.
  • Longley, W.L., Solis, R.S., Brock, D.A., Malstaff, G., 1994. Coastal hydrology and the relationships among inflow, salinity, nutrients, and sediments. In: Longley, W.L., (Ed.) FreshwaterIinflows to Texas Bays and Estuaries. Texas Water Development Board, Austin, Texas, 23-72.
  • Morton, R.A., 1977. Historical shoreline changes and their causes. Transactions-Gulf Coast Association of Geological Societies 27, 353-363.
  • Morton, R.A., 1979. Temporal and spatial variations in shoreline changes, Texas Gulf Coast. Journal of Sedimentary Petrology 49, 1101-1111.
  • Morton, R.A., 1993. Shoreline Movement Along Developed Beaches of the Texas Gulf Coast: A User's Guide to Analyzing and Predicting Shoreline Changes. Bureau of Economic Geology, University of Texas at Austin, Open-File Report 93-1. 79 pp.
  • Morton, R.A., Paine, J.G., 1990. Coastal land loss in Texas-an overview. Transactions- Gulf Coast Association of Geological Societies XL, 625-634.
  • Morton, R.A., Purcell, N.A., 2001. Wetland subsidence, fault reactivation, and hydrocarbon production in the U.S. Gulf Coast region. United States Geological Survey Fact Sheet FS-091-01.
  • Phillips, J.D., 2001. Sedimentation in bottomland hardwoods downstream of an east Texas dam. Environmental Geology 40, 860-868.
  • Phillips, J.D., 2003a. Toledo Bend reservoir and geomorphic response in the lower Sabine River. River Research and Applications 19, 137-159.
  • Phillips, J.D., 2003b. Alluvial storage and the long-term stability of sediment yields. Basin Research 15, 153-163.
  • Phillips, J.D., Marion, D.A., 2001. Residence times of alluvium in an east Texas stream as indicated by sediment color. Catena 45, 49-71.
  • Phillips, J.D., Slattery, M.C., 2006. Downstream trends in discharge, slope, and stream power in a lower Coastal Plain river. Progress in Physical Geography 30 (4), 513-530.
  • Phillips, J.D., Slattery, M.C., Musselman, Z.A., 2004. Dam-to-delta sediment inputs and storage in the lower Trinity river, Texas. Geomorphology 62, 17-34.
  • Pulich, W.M., White, W.A., 1991. Decline of submerged vegetation in the Galveston Bay system: chronology and relationships to physical processes. Journal of Coastal Research 7 (4), 1125-1138.
  • Rodriguez, A.B., Anderson, J.B., 2000. Mapping bay-head deltas within incised valleys as an aid for predicting the occurrence of barrier shoreline sands: an example from the Trinity/Sabine incised valley. Gulf Coast Assoc. Geological Societies Transactions 50, 755-758.
  • Rodriguez, A.B., Anderson, J.B., Bradford, J., 1998. Holocene tidal deltas of the Trinity incised valley: analogs for exploration and production. Gulf Coast Assoc. Geological Societies Transactions 48, 373-380.
  • Rodriguez, A.B., Fassell, M.L., Anderson, J.B., 2001. Variations in shoreface progradation and ravinement along the Texas coast, Gulf of Mexico. Sedimentology 48, 837-853.
  • Rodriguez, A.B., Hamilton, M.D., Anderson, J.B., 2000a. Facies and evolution of the modern Brazos delta, Texas: wave versus flood influence. Journal of Sedimentary Research 70 (2), 283-295.
  • Rodriguez, A.B., Anderson, J.B., Banfield, L.A., Taviani, M., Abdulah, K., Snow, J.N., 2000b. Identification of a 15 m middle Wisconsin shoreline on the Texas inner continental shelf. Palaeogeography, Palaeoclimatology, Palaeoecology 158, 25-43.
  • Thomas, M.A., Anderson, J.B., 1994. Sea-level controls on the facies architecture of the Trinity/Sabine incised-valley system, Texas continental shelf. In: Dalrymple, R.W., Boyd, R., Zaitline, B.Z., (Eds.), Incised-Valley Systems: Origin and Sedimentary Sequences. SEPM (Society for Sedimentary Geology), Tulsa, OK, pp. 63-82.
  • White, W.A., Calnan, T.C., 1991. Submergence of vegetated wetlands in fluvial-deltaic area, Texas Gulf coast. Coastal Depositional Systems of the Gulf of Mexico. 12th Annual Research Conference. Society of Economic Paleontologists and Mineralogists, Gulf Coast Section, Tulsa, OK, pp. 278-279.
  • White, W.A., Tremblay, T.A., 1995. Submergence of wetlands as a result of humaninduced subsidence and faulting along the upper Texas Gulf coast. Journal of Coastal Research 11 (3), 788-807.
  • White, W.A., Morton, R.A., Holmes, C.W., 2002. A comparison of factors controlling sedimentation rates and wetland loss in fluvial-deltaic systems, Texas Gulf coast. Geomorphology 44, 47-66.
  • Yeager, K.M., Santschi, P.H., Phillips, J.D., Herbert, B.E. 2002. Sources of alluvium in a coastal plain stream based on radionuclide signatures from the 238U and 232Th decay series. Water Resources Research 38: 1243, doi: 10.1029/2001WR000956.

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