Camping and sleeping under bridges harms everyone because of all the waste that ends up in watersheds. I've helped out with cleaning up riverbanks a couple times after illegal encampments were removed. Those places were like disaster areas and toxic waste dumps.
There's no trash removal for these people, and they're shut out of all public bathrooms. What do we really expect?
i-5 in Seattle is absolutely trashed from Northgate to Downtown because Jenny Durkan and previous mayors have raided homeless encampments out of view, away from other homes, spending millions of dollars to displace them instead of just giving them garbage cans and a few porta potties. They end up on i5 and in tent camps downtown and it looks like a dump.
I'm not familiar with the situation in Seattle, but in areas where homeless shelters have open beds then I really expect people to use them instead of wrecking ecologically sensitive areas. And in the cleanups I helped with, there were multiple public garbage cans within a couple blocks.
I live in Michigan, where PFAS are being found in water supplies all over the state. Companies did and still do dump poisonous chemicals into the environment, and day by day certain alphabet soup agencies give them more and more license to do so. We can talk about some garbage along a river bank after we prosecute large companies for knowingly poisoning entire swaths of land and bodies of water for decades, considering the scale on which they can harm people and the resources they have to avoid doing so.
But its only poor peoples' communities that get the brunt of the pollution. That's because their local governments can't fight against it, whereas richer communities can delay, fight, and stop.
Missing the point? My point is that a little trash on a riverbank pales in comparison to the large-scale environment destruction being committed every day by giant companies. Provide the homeless with food and shelter instead of worrying about some minimal pollution.