The Greenlawn Avenue Dam is a lowhead dam spanning the Scioto River south of downtown Columbus, Ohio.
Below the dam lies a wooded floodplain which regularly floods, especially in winter.
The floodwaters carry a significant amount of floating debris and trash. "Floating" means not only trash floating on the surface of the water; it includes a large amount of trash, mainly plastic bags and sheet plastic like tarps, with neutral bouyancy which lets it float unseen below the surface.
When high water passes through trees and undergrowth, the foliage snares this plastic, which is revealed when the waters recede. There's no way to know how much of this plastic passes by without being snared somehow, if not here then somewhere else downstream. Sometimes items like plastic grocery bags collect silt, and they sink, becoming embedded in the riverbank.They never decompose; the plastic eventually "photo-degrades" becoming brittle, and it breaks apart into smaller and smaller pieces.
This is a small island just below the dam. The plastic shows how high the water level was. This is mostly light plastic like grocery bags.
The high water brings in all kinds of floating material, mostly sticks and logs. The sticks back up among the trees, creating jams which capture more floating debris. The water flows through, but the debris is captured. The majority of snagged waste material comprises small items like plastic bottles (sometimes capped glass bottles) along with styrofoam cups and pieces, snack food bags, etc.
When the water recedes back to normal river level, the litter is left behind logjams that strongly resemble beaver dams. It sometimes looks like someone came in with a truckload of litter, and dumped it.
Our volunteers really hustle, and they take pride in making a visible difference in the park. In past years we would make a heap of trash up by the entrance to the park, but now we leave everything under the Greenlawn Avenue bridge, out of most public view and where the Columbus Recreation and Parks crews have more room to gather it up without blocking traffic.