I read something about this several months ago, though I’ve since lost the source, but here is another article that talks about the toxicity to adult salmon of the water that runs off roads.
They don’t know exactly what it is in the road runoff that is the problem; tests in labs with mixtures of oil and other chemicals didn’t seem to harm the fish. However, from both real-world observations, and tests in labs using actual runoff, it is clear that whatever’s in the mix is really lethal:
The research got its start more than a decade ago, when habitat-restoration projects began coaxing a trickle of coho back to several urban streams in the Puget Sound area. But many of those fish died before they could spawn. And the deaths seemed to coincide with rainstorms that sent runoff surging through drainage pipes and into the waterways.
In some place, like Longfellow Creek in West Seattle’s Delridge area, up to 90 percent of females were killed.
. . .
In experiments at the Suquamish tribal hatchery near Poulsbo, every coho exposed to the runoff died — some within a few hours, all within a day.
The good news is that it is relatively simple to prevent streams from being toxic, by creating bioswales or similar structures that filter water coming off the roads through soil.
But whatever the chemical culprit, the scientists found it could be removed by passing the runoff through 55-gallon drums packed with layers of gravel, soil and compost. None of the fish exposed to the filtered stormwater died or fell ill.
. . .
The finding is a strong endorsement of rain gardens, grassy swales and other “green” alternatives to traditional drains and pipes designed to collect stormwater.
The other article I read also talked about the importance of rain gardens and bioswales along roads near fresh water, because of their ability to reduce or eliminate the harm to fish from road runoff. While this article is specific to salmon, it would hardly be surprising if this improved the overall health of the water sources affected, along with everything else living in or near them.