Understanding Corpus Christi water levels
The two lakes behind the number
Corpus Christi does not draw its water from one lake. It draws from two, operated as a single system. Choke Canyon Reservoir sits on the Frio River near Three Rivers. Lake Corpus Christi sits downstream on the Nueces River near Mathis, and it is the lake the city actually withdraws from. Water released from Choke Canyon runs down the Frio, meets the Atascosa and the Nueces above Three Rivers, and works its way to the lower lake. Filled to the top of their conservation pools, the two together hold 918,882 acre-feet. An acre-foot is about 326,000 gallons, roughly enough to cover an acre of ground one foot deep. Choke Canyon is the larger and deeper of the pair and acts as the storage bank. Lake Corpus Christi is shallower, loses more to evaporation for its size, and acts as the working account.
Why the combined figure is the one that matters
The City of Corpus Christi does not set drought restrictions off either lake on its own. Its drought contingency rules trigger on combined storage, the two lakes added together as a percentage of the system. That is why the headline number here is a combined figure, and why a strong rise at one lake can be offset by a drop at the other without changing what residents are asked to do. The water restrictions page lays out the thresholds and what it takes to clear a stage.
What the percentage is a percentage of
The percentage is conservation storage divided by conservation capacity. Conservation storage is the water the system is permitted to hold and use for supply, not everything the reservoirs can physically contain. Above the conservation pool sits the flood pool, space kept deliberately empty so a heavy rain can be caught and let out in a controlled way. Water in the flood pool is not counted here, because it is not supply. It is water on its way back out. Two things follow from that. A lake can look full from the boat ramp while the percentage reads well under 100. And the combined figure can sit near 100 percent while releases continue, because the system is shedding water it is not allowed to keep.
The percentage also covers only these two reservoirs. Corpus Christi receives water from Lake Texana and the Colorado River through the Mary Rhodes Pipeline, and none of that supply appears in the figure at the top of this page.
Why the two lakes rarely move together
Rain in the upper basin does not reach both lakes the same way, and the gap between them is a permanent feature of this basin rather than a quirk of any one storm. Between the upstream gauges and the lower lake, water crosses long stretches of sandy riverbed. The dry channel and the shallow aquifer beneath it absorb a great deal of flow, so a rise measured at Tilden or Three Rivers arrives smaller, later, and sometimes not at all. Choke Canyon sits at the top of the Frio arm and captures its inflow directly, so it can gain from a storm that barely moves the lower lake.
Releases matter as much. Lake Corpus Christi receives Choke Canyon water only when a release is made, and those releases are typically small and made for specific reasons, not to equalize the two lakes. Through long dry stretches the lower lake leans on whatever the Nueces and Atascosa deliver while it keeps serving city demand. There is a page for each lake, with the river gauges that feed it: Choke Canyon and Lake Corpus Christi.
Where these numbers come from
Reservoir storage and river flow come from the U.S. Geological Survey, which operates the basin gauges. This site checks for new readings about every fifteen minutes, though gauges report at their own pace and a value can be an hour or more old on arrival. Readings are provisional until the USGS reviews them and are sometimes revised. When a gauge stops reporting or the USGS flags an equipment problem, this site says so next to the reading instead of showing an old value as current. The twelve-month history and the long-term averages come from Texas Water Development Board daily reservoir estimates, which is also what the drought stage tracking is walked against. All times are Central. For flood forecasts and warnings, see the National Weather Service.