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Lake Granger Watershed Assessment and Implementation Project

Project Goals/Objectives: Facilitate the development of a watershed protection plan for the Lake Granger Watershed; install best management practices for the purpose of reducing erosion within the watershed; reduce sediment loadings by 20-30% (based on RUSLE calculations or other applicable means); reduce TSS concentration in the reservoir by 30%, reduce nutrient loadings from agricultural lands and monitor for results.

This project will provide the Little River-San Gabriel Soil and Water Conservation District (LR-SGSWCD) and the Taylor Soil and Water Conservation District (TSWCD) with funding for technical assistance and financial assistance to implement best management practices through conservation planning. A district technician working out of the Little River-San Gabriel SWCD district office will provide the technical assistance for the project. The Brazos River Authority will conduct routine and stormwater monitoring to determine and quantify the effects of the project. The NRCS and Texas AgriLife Extension Service will provide their technical expertise by participating on the Advisory Group.

Project Background: The NRCS-Water Resources Assessment Team conducted the Volumetric Survey of Lake Granger in 1995 for the Brazos River Authority. The purpose of the survey was to determine the capacity of the lake at the conservation pool elevation and to establish a baseline for future surveys.

In 1980 when Granger Lake first started impounding water, initial storage calculations estimated that the volume of the lake at the conservation pool to be 65,510 acre/feet. The October 1995 survey determined the volume of the lake to be 54,280 acre/feet. The loss of 11,230 acre/feet over the 15 year life of the lake represented 17% of total storage at a rate of 748.67 acre/feet per year (1,343,095 tons of sediment /year).

In 2002 the Texas Water Development Board (TWDB) conducted a similar survey to determine the sediment loadings from October 1995 to April 2002. Results indicate a loss of 1,319 acre/feet in the 6 and ½ years between the surveys representing a loss of 202.92 acre/feet per year (364,033 tons of sediment/year). There is a distinct difference in the annual loss of volume in the lake between 1980-1995 and 1995-2002. These differences are directly related to rainfall and storm intensity.

In 1999 the NRCS-WRAT, at the request of the BRA, conducted a separate study of the Granger Lake watershed using the Soil and Water Assessment Tool (SWAT) basin model to assess flow and sediment loads and the effects of various BMPs to those sediments. Modeling results indicated that a combination of conventional conservation practices has the potential to reduce sediment loads by 20-30%. Other practices such as stream bank restoration and created wetlands were not taken into account in the study.

Williamson County is currently the fourth fastest growing county in the country. Lake Granger currently serves as a drinking water supply reservoir for approximately 20,000 residents of Williamson County. The public drinking water demands on the lake are expected to increase to exceed 100,000 residents in the next five to ten years. As the population continues to grow in this area, it becomes increasingly important to protect and preserve the water quality of Lake Granger. 1999 land use photography indicated that approximately 33% of the watershed is Brushy rangeland, 24.5% is open rangeland, 20.5% is row crop agriculture, and 11% is pasture and hayland, and the remaining 11% is Urban. While only 20.5% of the watershed is row crop, the soils and proximity of the cropland to the lake make it the main contributor to sediment loadings. Technical and financial assistance are needed to assist landowners with implementing best management practices that will reduce agricultural runoff.

The BRA owns and operates the East Williamson County Regional Water System that treats water from Lake Granger. Sedimentation into the reservoir not only threatens to reduce the firm yield of the reservoir but also causes significant problems in treating the water. Turbidities above 200 ntu (Nephelometric turbidity units) are common with turbidity spikes exceeding 5000 ntu. Treating these high levels of turbidity to achieve the 0.1 ntu drinking water standard results in increased treatment costs that must be passed on to customers.

Project Location: Granger Lake, Segment 1247; Willis Creek, Segment 1247A; San Gabriel River, Segment 1248

Mankins Branch and Willis Creek are listed on the 2004 Inventory and List for bacteria and the San Gabriel River is listed for total dissolved solids. In addition, the growing importance of Granger Lake as a regional water supply has heightened a long-standing concern of excessive sedimentation to the reservoir. This WPP will address these water quality concerns and assist local stakeholders in developing long-term management measures to protect water quality and extend the life of the reservoir.

The point of contact for the project is Jay Bragg with the BRA (jbragg [at] brazos.org or (254) 761-3135.

Project Costs: Federal ($814,168); Non-Federal Match ($413,889); Total ($1,228,057)

Project Participant(s): TSSWCB, Brazos River Authority, Little River-San Gabriel Soil and Water Conservation District #508, Texas AgriLife Research - Blackland Research and Extension Center

Project Workplan: 05-09 (PDF, 26.7 kB)

Quality Assurance Project Plan: 05-09 (PDF, 525 kB)

Project Final Report: 05-09 (PDF, 17.6 MB)