Yes, rivers are ideal places to study microplastics for a couple of reasons. First, you can gain baseline data about the state of the body of water, the same as we do in oceans and lakes. But even more importantly, rivers are also sources of lake and ocean plastics, so studying their plastic loads additionally gives data about plastic inputs into other areas.
There has been some interesting research on canals, especially urban ones used for storm drainage, since looking at them after rain events can tell a lot about the movement of plastics from the land into waterways.
Babylegs can be tied off on an overhang such as a bridge to monitor rivers, streams, and canals, rather than using a boat.