About Lake Corpus Christi
Lake Corpus Christi is the lower and working half of the supply system. It sits on the Nueces River near Mathis, behind Wesley E. Seale Dam, and its conservation pool holds 256,062 acre-feet when full. Its official name is Lake Corpus Christi, though locally it is just as often called Lake Mathis, after the nearby town. When people ask what the lake level is, they usually mean the surface elevation, the height of the water in feet above sea level, which is the elevation figure on the card above. This is the lake the city draws from: water leaves here for the Calallen pool and the treatment plant, which makes its level the more immediately felt of the two. It is also the shallower of the pair, with a wide surface relative to its volume, so it gives up more to evaporation in a South Texas summer than its size alone would suggest.
Almost everything reaching this lake passes the Three Rivers gauge first. Upstream of it, the Nueces arm runs from below Uvalde through Asherton, Cotulla, and Tilden, and the Atascosa arm comes down from Whitsett. The Frio joins from the west carrying whatever Choke Canyon has released. All three meet just above Three Rivers, which is why that single gauge is the best measure of what is actually on its way to the lake, and why the arms are worth watching separately when you want to know where a rise is coming from.
A rise upstream does not translate into a rise here one for one. Between Three Rivers and the lake are about 50 river miles of sandy channel, and a dry riverbed with a depleted shallow aquifer beneath it takes its share before the rest arrives. Modest flows can be absorbed almost entirely, while a large sustained flow, having wetted the channel, gets through with far less loss. That is the main reason this lake can hold steady or keep falling while gauges upstream show water moving, and the reason the two lakes so often read differently at the same moment.