I was recently in Richardson, Texas, along the stream of an urban mini-park. Mid-fall the water was low, a pleasant trickle. Spring high water was easily seen in the unending string of plastic sacks hooked to twigs, leaves, branches as far as the eye could see. It looked like a third-world dump, or a ghastly Halloween site. 
So, don’t use ’em. Pick ’em up when you seem ’em. Tie ’em in knots. Return to sender….
San Francisco, by the way, has banned them starting today.