As you know, paddlers share rivers with lots of users who may not share our appreciation of this resource.  I keep a lawn trash bag in my boat during day trips, which allows me to collect the styrofoam bait buckets, ubiquitous plastic bottles, and other small stuff.  I appreciate canoe liveries that clean up the bigger stuff, like tires, grocery karts, and other junk that makes it into our rivers, even in the most remote areas.  The one area that is difficult to address is sewage plant outflows.  On a recent trip down one of Ohio's "natural and scenic rivers," we ran into an untreated outflow from a newer plant that was not supposed to pollute the river.  It was disgusting.  All sewage plants have untreated discharges at times.  We should covet all streams (and portions thereof) that don't currently have treatment plants, as clean water is a disappearing thing in most areas of the country.