An impeller pump is essentially a fan blade that forces water through a pipe from the creek side out into the mainstem river. As one might imagine, this is not fish-friendly at all! Fish mortality with these types of pumps can be quite high, as fish get chopped up and killed, or injured severely, making them prone to predation and disease. There can also be drastic pressure changes inside the pump that can pop the swim bladders of fish. Not shown in this animation but typical to most stations, the pump is paired with a floodgate, but the gate closes when the water levels in the mainstream river get too high. Once the gate closes, water and fish are forced through the pumps. Though fish-unfriendly designs are much more common, there are fish-friendly versions of impeller pumps that have larger spaces between the blades, rotate at slower speeds, and are housed in cages to prevent fish getting caught between the blades and the sides of the pump station.