Once-through cooling (OTC) systems may be used on very large rivers or at coastal and estuarine sites. These power stations put the waste heat into the river or coastal water. These OTC systems thus rely upon a good supply of river water or seawater for their cooling needs. Such facilities are built with intake structures designed to pump in large volumes of water at a high rate of flow. These structures tend to also pull in large numbers of fish and other aquatic organisms, which are killed or injured on the intake screens. Large flow rates may immobilize slow-swimming organisms including fish and shrimp on screens protecting the small bore tubes of the heat exchangers from blockage. High temperatures or pump turbulence and shear may kill or disable smaller organisms passing the screens entrained with the cooling water. Over 1,200 power plants and manufacturers use OTC systems in the U.S. and the intake structures kill billions of fish and other organisms each year. More agile aquatic predators consume organisms impinged on the screens; and warm water predators and scavengers colonize the cooling water discharge to feed on entrained organisms.