The last two years, I've been involved in organizing the Steelhead Society cleanup in the Squamish watershed in April two years running. Organizing takes some work, but the results are extremely satisfying. In the past two years, we've moved somewhere in the order of 18,000lbs of garbage from the river bank to the dump.
Try getting ahold of the district, you will likely find them happy to provide bags, gloves etc. You should be able to get them to waive tipping fees when you visit the landfill. With the Squamish Cleanup, we've had donuts brought by participants or donated by Tim Hortons as well as coffee from Starbucks in Squamish. We've also arranged to have Fergies Cafe, an awesome diner on the banks of the Cheakamus River, to cut volunteers a deal for a beer and burger for a pretty good price. Maybe you could arrange something similar with Sasquatch Inn near the Chehalis. Forums such as FWR are a great avenue to organize these events. I'm not that familiar with groups in the Fraser Valley, but we were able to get the Squamish River Watershed Society, Squamish Streamkeepers on board, they provided insight into broadening the focus from a cleanup to include other initiatives, such as possibly eel grass planting, bank stabilization of spawning areas etc. We're looking into broadening the focus next year as we're cleaning up garbage faster than its been deposited.
Interestingly, I've definitely noticed that once cleaned up, an area stays relatively clean for a while. People notice, they pick up after themselves and others.
Here's some photos from our Squamish Cleanup.
http://www.steelheadsociety.org/news/2012-squamish-watershead-cleanupThe two biggest players in the Squamish River guiding scene, Clint Goyette of Valley Fishing Guides, Shane and Justin from Whistler Fly Fishing and SSBC director Carlo unload a trailer of garbage into the dump.
We were able to turn areas that looked like this before:
Into: