Indications and potential sources of change in sand transport in the Brazos River, Texas

Changes in the capacity of the Brazos River to transport sand can be identified within the context of Lane’s relation through changes in channel geometry, changes in the characteristics of suspended loads, and changes in discharge. The Brazos River channel has been undergoing continual adjustment since the 1940s. For a discharge of 5,000 cubic feet per second, the watersurface altitude has decreased 2 to 4 feet at the Hempstead and Richmond streamflow-gaging stations between 1940 and 1995. The characteristics of suspended-sediment samples at the Richmond streamflow-gaging station have changed between the periods 1969–81 and 1982– 95. The amount of sand-size sediment transported in suspension has decreased. The distribution of both daily and annual-peak discharges has changed. However, the computed annual loads of suspended sand indicate no statistically significant change in the median annual load.

The transport of sand in the Brazos River depends on a complex set of factors, most of which are continually changing. Potential sources of change in sand transport in the Brazos River include the effects of reservoir construction, changes in land use, and instream sand and gravel mining. Extensive reservoir construction in the Brazos River Basin has reduced sand transport by trapping sediment and by reducing the magnitude of peak discharges. However, reductions in sand transport associated with reservoir construction apparently are compensated for by increases associated with tributary sediment inflow and localized bank erosion. The total area of harvested acres of non-hay crops in the lower Brazos River Basin during 1924–92 decreased more than 75 percent from about 32 percent to about 8 percent of the total area. Correspondingly, erosion potential has decreased substantially. Several sand and gravel mining sites are located on the Brazos River between Hempstead and Rosharon. The quantity of sediment extracted by instream sand and gravel mining operations could represent from 11 to 25 percent of the total sand transported by the Brazos River. The effects of mining on sand transport could not be quantified.