The Columbia River is a major route for Coho Salmon. There are highways on both sides of the river from the mouth of the Columbia to hundreds of miles inland. I-80 especially is a heavily trafficked road since it’s one of the few E-W highways through the Cascade range. And this is in a region that has rain the majority of the year. I assume tons of this chemical is being washed into the river each year. I’m not sure what can be done about it. Shut down the highways in Washington and Oregon along the river? Cut a new highway from Boise to Portland through the Cascade range somewhere south of the Columbia? Somehow restrict vehicular traffic along those highways? The cheapest and simplest solution might be to build some sort of catchment system along the roads that collect and cleans all runoff water. It would cost billions, but would be cheaper than building a new highway through the Cascades.