The H-GAC region includes 13 counties in southeast Texas. The City of Houston, the fourth largest city in the United States, is the center point of the region. Parts of the San Jacinto River Basin boundaries spill into two adjacent counties. The region includes four basins containing a total of 39 watersheds encompassing 51 TCEQ Clean Rivers Program Segments.

The H-GAC's soil, vegetative and mineral diversity is a result of the region's subtropical climate and fluvial geologic characteristics. The area receives an average of 45 inches of rain each year, with a strong influence from the Gulf of Mexico. Topography ranges from just over 400 feet in the northern counties, to sea level at Galveston Bay and the Gulf coast. Surface water bodies include streams, rivers, bayous, lakes, reservoirs, estuaries and the open waters of Galveston Bay and the Gulf of Mexico. Flow from the bayous into Galveston Bay is generally sluggish due to the gently sloping topography.

Without the inputs of treated effluent, many of the creeks and bayous would not normally flow year-round, although there are few springs that contribute significantly to baseflow. Due to shallow groundwater tables, there is influence of groundwater recharge to some channels. Soils are predominantly clayey with dispersed areas of sandier substrate near and around river channels. Distinct riparian vegetation can be found along river floodplains. Mineral resources include oil and gas fields, lignite, sand and gravel, clay, salt and sulfur.

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