Run-of-river (not creating a large dam), low head (small elevation difference between water inlet and outlet) low rpm (slow turning wheels) have several fish and wildlife friendly features. Fish ladders, to allow upstream migration of swimmers are relatively short and do not present themselves as a significant barrier or cost to a project.
Most importantly, the low velocity of the wheel or Archimedes screw turning means that any fish, eels or other wildlife that get caught in the wheel just float down and out the other side. This is explained quite simply in the introduction to the H2OPE FAQ. Most modern and efficient water turbines are designed to operate a high speed which can kill fish and wildlife if not protected by costly and mechanically complex protective screening mechanisms.
Also, a swiftly running stream may present little opportunity for habitat and shelter. Fish and other wildlife need and thrive in ponds where rushes and other stream side vegetation and habitat are allowed to grow. By created periodic still waters and mill ponds habitat is increased and the complexity of the ecosystem can be multiplied as migrating and resident waterfowl can nest.
Thirdly, and certainly not least, a well managed and maintained site should be capable to removing a certain amount of detritus and debris that may enter our waterways as an undesired outcome of modern industrial life. Many existing examples of hydro stations extract a lot of urban trash, plastic bottles, toys and other floating debris that is harmful to the environment and wildlife in general.
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