A fish screen is designed to prevent fish from swimming or being drawn into an aqueduct, cooling water intake, intake tower, dam or other diversion on a river, lake or waterway where water is taken for human use. They are intended to supply debris-free water without harming aquatic life. Fish screens are typically installed to protect endangered species of fishes that would otherwise be harmed or killed when passing through industrial facilities such as steam electric power plants, hydroelectric generators, petroleum refineries, chemical plants, farm irrigation water and municipal drinking water treatment plants. However, many fish are killed or injured on screens or elsewhere in the intake structures. Design Industrial facilities Fish screens may be positive barriers (devices such as a perforated metal plate that physically prevents fish from passing) or behavioral barriers (devices that encourage fish to swim away). Most behavioral barriers are experimental and of unproven effectiveness. Positive barriers are often effective at keeping aquatic organisms from entering a cooling system, but may also kill them by impinging them on the screens. These barrier types are widely used and include: Modified traveling screens Fish handling and return systems Horizontal, flat-plate screens with bypass water return systems Cylindrical wedge wire screens Fine-mesh screens Fish net barriers. Besides simply preventing fish from passing, fish screens are designed to minimize stress and injury that occur when fish impact the screen or are subjected to changes in water velocity and direction caused by the diversion. The U.S. Environmental Protection Agency (EPA) has evaluated other barrier technologies and identified some as potentially effective, although not widely demonstrated (as of 2004): Aquatic microfiltration barriers Angled and modular inclined screens Velocity caps. Some fish screens are designed to protect a single species of fish (for example, salmon) and are not necessarily effective at protecting other fish species. Some screens are capable of protecting more than one species or type of life. Additionally, some screens may effectively protect juvenile and adult fish, but not fish eggs and larvae. The cost of a fish screen varies from thousands of US dollars for small, low-flow-rate screens to millions of US dollars in the case of very large custom-designed systems that filter a large flow of water. Maintenance costs can be significant, including repairs, removing trash, and adjusting the equipment for changes in stream conditions. Non-industrial applications For some low volume, non-industrial water diversion applications, there are screens available