The 48-hour bacteria rule

Rain is what makes the Greenbelt swimmable, and, briefly, what makes it unsafe. Storm runoff washes everything on the watershed's surface into the creek: soil, pet and wildlife waste, and the E. coli that comes with it. Bacteria counts spike during and right after rain, then decay as sunlight and dilution do their thing.

The standard local guidance, echoed by the City of Austin's watershed staff and LCRA: wait 48 - 72 hours after significant rain before swimming in creeks and lakes, and up to a week after a major flood. Barton Springs Pool itself often closes after heavy rain for the same reason (plus debris and visibility for lifeguards).

Our countdown automates that rule. We watch the rain gauges across the Barton Creek watershed and call rain "significant" when any of them accumulates half an inch or more within 24 hours. The clock starts at the end of the last significant rain: for the first 48 hours we show caution, from 48 to 72 hours improving, and past 72 hours clear.

This is a heuristic, not a measurement. The city's lab data on E. coli runs a month or more behind, so nobody has real-time counts. Also, the best swimming usually arrives just as the caution lifts: 3 - 7 days after rain, when the flow is still fresh but the flush has passed. The countdown isn't there to scare you off. It's there to tell you when the good window opens.