Dams are major obstacles to the movement of fish, either upstream or downstream. Although the free movement of fish can sustain a healthy fishery, a dam may also be a barrier to the movement of unwanted invasive species. For example, the Orienta Dam on the Iron River in northwest Wisconsin prevents the further movement of sea lampreys up the Iron River from Lake Superior. The dam removal proposal for this location recommends a barrier designed to restrict movement of unwanted species. Just as different types of fish prefer different temperatures, so do different types of invertebrates.
Many invertebrates rely on the river s current for supplying food or for respiration.
Different aquatic invertebrates also prefer different types of streambed materials. Mussels are sensitive to high turbidity. Riverine insects generally require more dissolved oxygen than lake-dwelling insects. Aquatic plants and invertebrates provide most of the food source for fish in free-flowing rivers. By seasonal movement of fish, a single river may support fish preferring all ranges of temperatures at different times of the year or at various life stages.