Return flows from irrigation projects may be warmer, sediment laden, and contaminated with chemicals, including biocides and fertilizers. Conversely, return flows into river channels during droughts can provide some beneficial effects. Flows from hydroelectric plants may fluctuate greatly and create unstable environments. Water degraded in temperature or chemical composition can displace, or limit growth in fish populations. However, projects providing planned flows can mitigate and potentially enhance fish populations in some environments.
Estuaries are also sensitive to alteration and reduction of stream flow.
Eggs and larvae of some estuarine fishes cannot tolerate high salinity of the marine environment that may result from reduced freshwater flows. In this case, the quantity and seasonal timing of freshwater inputs are particularly critical to these sensitive stages. Productivity of estuarine food organisms partially depends on the alochthonous material transported by rivers. Reservoirs may act as nutrient traps thereby reducing estuarine productivity. Instream flows are a public trust, and stream ecosystems must be protected as irreplaceable resources.