Altered Hydrology. While dewatering is not considered a threat in the Trinity Basin during the 10-year timeframe of this agreement (as discussed in Section 4), changes to water quantity and altered hydrology are considered ongoing threats for the Covered Species. The Trinity River Basin provides water for the DFW area in the upper part of the basin and the Houston metro area in the Lower Basin. Upstream infrastructure related to flood control and water supply reservoirs in combination with constant regional wastewater return flows from treatment plants that are further bolstered by inter and intra-basin water transfers into the Upper Basin, contracted bed and banks delivery of water, downstream senior water rights, and geography, ensure that under typical baseflow and dry weather/drought conditions there is more water available than would be present under natural conditions (Figure 14) (Land et al., 1998; Austin, 2006; TRA, 2017; Xxxxxxx, 2018; Xxxxx and Xxxxxxx, 2019). These water supply and wastewater activities have resulted in baseflows that have steadily increased over the past century from less than 160 cfs to over 620 cfs (based on annual minimum 3-day mean discharge). While TRA’s water supply obligations and wastewater return flows help ensure base and low flows are sustained in the river, these same flows may also exceed conditions that mussels or other species can tolerate (e.g., water velocity and shear force), resulting in degraded habitat that may no longer be suitable for freshwater mussels or other Covered Species in the Upper Basin especially. While elevated baseflows could alter habitat, it is likely that other major operations that are outside of the Covered Parties’ control have far greater influence on habitat quality. For example, the USACE operates six major flood control reservoirs and the Dallas Floodway in the Upper Basin and two flood control reservoirs in the Middle Basin. Most of these reservoirs have been in place since the 1950s and 1960s. These reservoirs capture and store excess water during periods of heavy rainfall, then release that water in a controlled manner after flood risks have subsided. The release schedule lowers the peak flows but extends the amount of time that the channel remains full. This altered hydrologic regime can cause increased erosion downstream of reservoirs due to the extended periods of higher-than-normal flow and reduced deposition of sediments that are captured upstream or entrained by the reservoirs. This altered...
