STOP 1: THE BRAZOS RIVERThe Brazos River west of Houston is a classic example of a meandering river. The Brazos River floodplain west of Houston is 8 to 10 miles wide and includes all of the characteristic features of a meandering river system. Figure 1 shows a bend of the Brazos River upstream from the Grand Parkway Bridge near Richmond. A large point bar can be seen to the left of the river channel. To the right of the channel is the cut bank. The river is eroding the cut bank as the meander loop migrates outward to the northeast. |
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Characteristic FeaturesThe characteristic features of a meandering river system are a highly sinuous channel, point bar deposits with opposing cut bank, and a wide floodplain containing abandoned channel segments. Figure 2 shows the major features of a meandering river floodplain. |
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| Rivers are important parts of the hydrologic system because they return water to the ocean. Rivers are also conduits for transporting sediment. The ability of a river to move sediment depends on the velocity of water in the river and the size of the sediment particles. Water moving at a low velocity can move only small fine particles such as clay, silt and sand. Water moving at a high velocity can move large coarse particles such as cobbles and boulders as well as small particles. Most river channels contain water flowing at several velocities. Under normal flow conditions coarse sediment such as gravel is deposited in the deeper part of the channel called the talweg and the fine sediment such as silt and clay are deposited in the shallow water on the slopes of the point bars. | |
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Sedimentary StructuresFigure 3 shows the bedding pattern of sand in a small excavation into a point bar. When the particles come to rest as sediments on the bottom of the river they form distinctive patterns. The shape and size of these sedimentary structures are determined by the size of the particles, the velocity of the water flowing over them, and the direction of flow. Characteristic bedding patterns associated with point bars are large scale trough cross bedding, horizontal bedding and small scale trough cross bedding. |
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| Sediment grain size in typical meandering river deposits decreases upward from gravel at the bottom to clay at the top. Sand from this point bar shown in Figure 3 is being mined for use in construction projects in the Houston area. | |
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Floodplain FeaturesWhen the river is in flood stage it river may leave its banks and cover parts of the adjacent floodplain. As the flood waters recede clay and silt are deposited on the floodplain. If flood stage conditions persist the river may abandon the existing channel and cut a new channel across the neck of a meander loop. The abandoned channel may then become isolated from the river and fill with water to become an oxbow lake. Figure 4 shows an example of an oxbow lake in Brazos Bend State Park. This oxbow was once an active segment of Big Creek, a tributary of the Brazos River but was cut off and abandoned. More examples of oxbow lakes can be seen along Highway 6 in the First Colony development in Sugar Land. |
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Access to the SitesFigure 5 shows the location of several points along the Brazos River near Houston where visitors can view the features of a meandering river. Public access to the river is available at the Brazos River Turnaround on U.S. Highway 59 northbound from Victoria. From the parking area under the bridge various trails lead down to the river and along the riverbank. The trail upstream leads to a large point bar which is more than 2500 ft wide and more than 40 ft thick. At Brazos Bend State Park, Hale Lake in the east part of the park is an example of an oxbow lake. The Brazos River can be accessed via the Red Buckeye and Whiteoak Trails located on the east side of the park.Inda Immega 25.Oct.98 |
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For More Information:The following references provide a detailed discussion of the significant features of the Brazos River in the Houston area:Bernard, H. A., and others, 1970, Recent sediments of southeast Texas - a field guide to the Brazos alluvial and deltaic plains and the Galveston barrier island complex; Bureau of Economic Geology Guidebook 11, The University of Texas at Austin. Morton, R. A., and McGowen, J. H., 1980, Modern depositional environments of the Texas coast; Bureau of Economic Geology Guidebook 20, The University of Texas at Austin, 167 p. Contributors:Site documentation - Houston Geological Society Field Trip Committe members Jim Saye, Richard Howe, and Jeannine Perrot.Web pages - Computer Applications Committee members Inda Immega and
Bill Osten.
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