Buffalo Bayou looks almost pretty in the spring sunshine, just a few miles upstream from the Houston Ship Channel.
But Tessa McClellan with Environment Texas says the pollution is definitely there, under the surface.
"Right now our waterways in Texas are a polluter's paradise. The Houston Ship Channel ranks number one in the state and number 16th in the nation for total amount of toxic chemical discharges."
The new study analyses legal discharges into rivers and streams, self-reported by companies to the federal government.
Even so, McClellan says the chemicals are still very harmful.
"Some of the toxic chemicals that we're looking at include chemicals that cause cancer, like arsenic; chemicals that harm women's reproductive health, like benzene; and chemicals that harm the development of fetuses and babies, like mercury."
The pollution can potentially affect drinking water.
But the biggest immediate danger is eating contaminated fish.
Ed Parten is with Texas Black Bass Unlimited.
He says advisories about contaminated fish damage the state's $6 billion dollar recreational fishing industry.
"What perplexes me as a an individual and a fisherman is why our elected officials will allow industry to pollute and then we the citizens have to pay the consequences, health-wise and then usually dollar-wise to clean up what was allowed to happen."
Environmentalists are backing an effort by the EPA to strengthen the Clean Water Act.
Because of recent court decisions, more than 75 percent of waterways in Texas are now excluded from the law's protection.