TSS Study on West Fork San Jacinto River (USGS)

Significant urban and industrial development, including sand and gravel mining, is ongoing in the West Fork San Jacinto River watershed between Lake Conroe and Lake Houston. Prior to 2008, Houston Public Works Department, the USGS and other interested stakeholders documented increased sediment in the upper west reaches of Lake Houston, including intermittent plumes of heavy sediment flowing down river, especially after rain events. The agencies speculated development activities were a primary source of increased sedimentation. However, existing Total Suspended Solids (TSS) data for the area neither supported nor disproved this assertion.

Clean Rivers Program routine ambient monitoring data is collected at a frequency of at least 30 days between each sampling event, and monitoring is not generally conducted during storm events when high sediment loads would be expected. The likelihood of random samples capturing an event is minimal. Subsequently, H-GAC contracted with USGS to study the suspended sediment (SS) and TSS in a selected reach of the West Fork San Jacinto River.

From July 2008 to August 2009, the USGS operated two continuous monitoring stations on the main channel of the river - one 2.5 miles below the Lake Conroe dam and another 2 miles downstream at I-45 - and measured specific conductance, turbidity and instantaneous flow every 15 minutes. Additionally, discrete water quality samples and flow measurements were collected during various flow conditions at the two main channel monitoring stations plus five selected tributary locations. Results were used to quantify suspended sediment concentrations and loads improving our understanding of the relationship between suspended sediment, turbidity and other real-time data. Sample results from the five tributary locations also allowed the USGS to evaluate loading from individual tributaries, helping to identify the source of the loading.

In all cases, the downstream site SS and TSS concentrations were approximately 40 percent greater than at the site below the dam. The USGS developed surrogate regression equations to estimate SS and TSS loads using real-time turbidity and stream flow data; however, the regressions have a relatively high percent error because of the minimum data set. As a result of the time required for the contract and QAPP development and prevalent dry weather conditions, USGS collected relatively few samples from the middle and upper flow ranges during the 14-month study period.

Using the regression curves, USGS estimated SS load to the river was 3,500 tons at the upstream site and 156,000 tons at I-45 while the total TSS load below the dam was estimated to be 1,900 tons compared to 72,000 tons at I-45. The extreme difference in total loading between upstream and downstream sites was not caused by continuous discharges but rather, episodic events frequently associated with rainfall. The continuous monitoring is ongoing but additional measurements are needed to help minimize the percent error found in the regression curves.

The concern over sedimentation to the river was exemplified in 2010 when the Bayou Land Conservancy, a land preservation organization based in Houston (formerly known as Legacy Land Trust), reviewed aerial imagery of the 100-year floodplain between Texas Highway 242 and US Highway 59 and determined that acres affected by sand and gravel mining operations had increased from approximately 1,300 acres in 1995 to nearly 3,700 acres. Consequently, without effective storm water management for sand and gravel mining operations and urban development, sediment loading to the river and, ultimately, Lake Houston, will continue to be detrimental to benthic habitat, navigation and the cost of water treatment for drinking water.

See the full study here.  (PDF 3.26MB)

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