It is also one of the most polluted rivers in the world. A study announced earlier this year found that the level of microplastics in the Tennessee River is among the highest ever measured. These small particles are known to accumulate in the gills and stomachs of ocean fish, and to move up the food chain when a larger fish eats a smaller one. There are far fewer studies of the impact of plastics in freshwater ecosystems, but early investigations suggest that the same principles apply. And we are the creatures at the top of the food chain.