The types and abundance of fish found in impounded systems depends on the characteristics of water found in impoundments. Different types of fish prefer different temperatures of water, especially for spawning. Many fish have narrow ranges of water temperature at which spawning occurs. Because impoundments may have relatively warm temperatures, warm-water species tend to dominate impoundments. An impoundment with cold water, however, will support cold-water species.
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.