Sediment in the San Antonio River Below San Antonio, Texas, 2000–13

The study area: 2,150 square miles of the San Antonio River Basin downstream from San Antonio. USGS.

The study

Sediment shapes the ecological health of rivers and estuaries. With the San Antonio River Authority, the USGS studied the basin downstream from San Antonio, Texas, in two parts:

1. Measuring sediment — suspended and bedload sediment, particle sizes and turbidity — from January 2011 to May 2013, combined with historical data from 1966–2004;
2. Modeling streamflow and suspended-sediment loads for 2000–12 with a calibrated watershed model.

How sediment was measured

Sediment moves two ways: suspended in the water, or as bedload rolling along the bottom.

• 67 suspended-sediment samples at 10 sites and 22 bedload samples at 6 sites, at flows from 1.9 to 10,600 cubic feet per second.
• Suspended samples were taken at 10 or more equal-width points across the stream, by wading or, where too deep, from a bridge or boat.
• Measuring flow at the same time turned concentrations into loads in tons per day.

Sampling suspended sediment by wading. USGS.

A BL-84 bedload sampler, which collects sediment resting on the streambed. USGS.

What they found

Almost all sediment is suspended. Most of it is fine silt and clay that stays in the water. On the San Antonio River near Elmendorf, bedload was only about 0.2 percent of total sediment by weight in 2011–12.

Rating sediment by flow. Regression equations relating measured concentrations to daily flow gave continuous daily loads at five sites — the San Antonio River near Elmendorf, near Falls City and at Goliad, Cibolo Creek near Falls City and Ecleto Creek near Runge — to check the model.

Suspended-sediment concentration versus streamflow, San Antonio River at Goliad; the fitted curve levels off where more flow no longer adds sediment. USGS.

The model

The Hydrological Simulation Program–FORTRAN (HSPF) simulates runoff, infiltration, channel flow, evapotranspiration and recharge, and the sediment they carry, anywhere in the watershed. Earlier HSPF models of the lower river were adapted for this study.

Measured and simulated daily streamflow at Goliad, 2006–12. USGS.

Estimated and simulated daily suspended-sediment load at Goliad, 2006–12. USGS.

Where the sediment comes from (2006–12)

• Coming in: about 737 tons a day from the San Antonio River near Elmendorf and 22 from Cibolo Creek near Selma — 759 in all.
• Going out: about 1,230 tons a day where the San Antonio meets the Guadalupe River — so the study area itself added about 471 tons a day.

Cibolo Creek contributes the most, likely because of steeper terrain and more developed land and cropland, which produce more runoff. Other years would give different loads.

Average daily suspended-sediment loads entering, generated within and leaving the study area, 2006–12. USGS.

Sources

Based on Darwin J. Ockerman, J. Ryan Banta, Cassi L. Crow and Stephen P. Opsahl, "Sediment Conditions in the San Antonio River Basin Downstream from San Antonio, Texas, 2000–13," U.S. Geological Survey fact sheet, prepared with the San Antonio River Authority; a work of the United States government in the public domain. The map, photographs and charts are taken from the fact sheet's PDF. In one place the fact sheet gives the incoming load as 757 tons a day; its own figures (737 + 22) and its earlier statement give 759.