Upper Basin. Conservation Zones A and B The Trinity River is comprised of four main tributaries that converge just below the DFW area. The Upper Basin’s hydrology has been highly modified and urbanized since the 1800s. Each of these tributaries contain at least one major multipurpose reservoir, and the river has been straightened and leveed for flood control purposes in many places. The USACE operates seven flood control reservoirs in the Upper Basin. These flood control reservoirs are operated as a system, which is designed to impound water in the flood pool (the reservoir capacity above normal conservation pool designed to temporarily store and release floodwater) and release that water in a controlled manner to prevent downstream flooding. There are also water supply reservoirs in the Upper Basin that do not have flood storage capacity and are not authorized to impound flood water. These reservoirs are designed to impound water up to the top of the conservation pool to ensure that water supply is available during dry times. Because these reservoirs do not have flood storage, water is captured until the reservoir reaches conservation pool elevation, then the remainder of the flow is released through xxxxxxx xxxxx downstream. Additionally, several of these reservoirs receive imported water from inside or outside of the Trinity River Basin to supplement their natural yield. While flooding and flood releases drive the hydrology during wet seasons, wastewater return flows supplement baseflows year-round and provide the majority of the water in the river during times of drought or low precipitation. Most of the water use in the Upper Basin is municipal and not wholly consumptive, that is, generally 63% of the municipal and industrial water used in the Upper Basin is returned to the river through WWTPs (Xxxxx Consultants, Inc., 2001). These wastewater return flows keep baseflows artificially elevated at all times but provide the only means of flow connectivity during times of drought in the Upper and Middle Basins.
