Swimming in Lady Bird lake is not allowed because of old construction material at the bottom and a drowning that happened 50 years ago not because of regular water quality. (https://www.kut.org/austin/2018-06-13/yup-its-still-illegal-to-swim-in-lady-bird-lake-heres-why)
We always have the water tested if there is any reason for concern, like recent storms. Usually, we even have water released from the dam so it is nice and fresh.