The Schuylkill River from the Girard Ave Bridge to Fairmount Dam has a significant plastic bottle accumulation along the West bank (link, link). Geomorphology can help us understand why.
The Schuylkill, like many rivers, meanders with numerous bends as it makes its way from the headwaters to the Delaware River. These bends play an important role in the shape and life of the river. Bend outside banks tends to erode while the inside banks tend to form point bars.
Source: http://www.chartiersgreenway.net/hydrology.htm
Let’s take a look at the Schuylkill from just upriver of Girard Ave Bridge to Fairmount Dam.
You can see 2 bends in the River, the bend near Girard Ave Bridge and the bend by Boat House Row. The mud flats on the West bank of the River by the Girard Ave. Bridge have been formed by point bar type deposition of sediment on the inner bank of the River. These mud flats have proven to be an effective natural plastic bottle trash trap, as shown in this photo of the West Bank taken on 4/11/15.
Most of these bottles have been carried by stormwater to the Schuylkill and have flowed downriver. During high flow periods some bottles accumulate along the inner curve of the River bends and get stranded when the River flow recedes.
The Girard Ave mud flats are an excellent location to monitor the Schuylkill River plastic trash load over time. The Schuylkill Navy has cleaned approximately 1,300 feet of the West bank so that future plastic accumulation rates can be measured at this location.