Wikipedia
A fish wheel is a device for catching fish which operates much as a water-powered mill wheel. A wheel complete with baskets and paddles is situated on the river, usually upon a floating dock. The wheel rotates due to the current of the stream pressing the bottom basket or paddle that has swung down into the water. The baskets on the wheel, swinging downstream only at the speed of the current or more slowly, capture fish actively swimming upstream and also can capture fish simply drifting.
As baskets rise towards the top of the wheel, lifted by the force of the river, the fish are shunted to one side and out an opening in the side of the basket, by gravity. This happens because the basket's inside edge, along the axle, is tilted at an angle relative to the axle and forms a slide to one side when the basket rises to vertical, as if it were a children's play slide standing lengthwise upon the axle. Therefore, as the basket reaches and passes vertical, fish flow to the inside edge and then along the slide to the side, spilling out an opening on the side of the basket. A holding tank is placed beside the wheel to receive the falling fish. A harvester need only come by only a few times each day to remove and process caught fish from the holding tank.
The yield is increased if fish swimming upstream are channeled by weirs into the path of the baskets, as was done on the Columbia River in Oregon, by large commercial operations which harvested vast amounts of salmon in the early 20th century, until banned. Fishwheels are operated in a different way by Tlingit Indians on the upper reaches of the Copper and Chilkat Rivers, harvesting salmon that are being pushed downstream by the current. In these settings the fish wheels harvest slowly and depend largely on luck to scoop under salmon resting in the current; more active means of harvest do far better.