However, too much plant growth actually cause problems for the fish and other animals. The abundance of plants will eventually deplete oxygen that is needed by fish and other aquatic life. Thick algae plumes also prevent sunlight from penetrating any significant distance into the water column, which inhibits the growth of underwater plants needed for food and shelter. Toxic pollutants include PCB's, dioxins, furans, chlorine, and heavy metals such as cadmium, chromium, copper, lead, nickel, and zinc. Some other concerns include oils, grease, antifreeze, paint, fertilizers, pesticides, bacteria, and litter.
Materials enter the river either through point sources or non-point sources.
Point sources discharge pollutants directly into the river via identifiable locations such as pipes from sewage treatment plants. It is possible to monitor and restrict these activities. Non-point sources are harder to identify and measure. These include erosion, spills, sewage overflows, litter, and water that runs over parking lots and roads which collects nutrients, soil, and pollutants which are deposited into the rivers. Even though it's harder to measure non-point sources, it's estimated that they pollute the river more than point sources.