Poop. That’s basically the answer.
Dallas is downstream of Fort Worth, where the water is fine for swimming.
But here in Big D, there’s too much pavement and too few regulations regarding runoff. Any time it rains, fertilizer and sewage flow into the river, which is chock full of E. coli.
WFAA has this interview with Woody Frossard of the Tarrant Regional Water District in Fort Worth, which requires new developments to filter all runoff from their properties.
The TV station also found that swimming in Lake Grapevine is maybe not a good idea.