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Disrupting Mating Season
Many aquatic animals, especially fish, have their reproductive cycles occur in accordance to annual flood seasons. Floods provide nutrients for both living organisms and the soil. Dams tend to disturb and alter the way these cycles happen, as dams may prevent floods or change the times floods occur. If a flood does not occur at a favorable season, aquatic offspring may be born in a season where their survival is unlikely.
Scouring and Removing Sediment
Dams may sometimes release water to transport it to canals or other water systems. This release will happen suddenly and in large quantities, as opposed to flooding in nature, which is more frequent and smaller in quantity. The sudden high-pressure release of water from dams can lead to the scouring of waterbed soil, removing sediment, plant life and even entire habitats on the waterbed.
Temperature Change
Whereas naturally occurring rivers rarely vary in temperature, the water in reservoirs have varying temperatures. Warm level sits on the surface in a reservoir, while increasingly cold water sits near the bottom. A release of water from these reservoirs will alter the temperatures of naturally occurring rivers. Micro-organisms depend on natural temperature cycles to survive, so dramatic temperature changes may put these creatures at risk.
Fish Migration
The release of water from dams also alters where fish migrate. Fish move upstream and downstream in accordance to their life cycles. Some dams are built with fish ladders, with which fish and climb over the dam walls. There are, however, risks inherent in this attempt, as fish sometimes die in their quest, falling downward to the river below. Fish can also be killed by nitrogen gas at the base of dams.