Karluk River Weir, Kodiak AK Photo by Tyler Polum
Why do fish bother
Swimming through the Weir? Why Not just turn around?
The brain of a fish is designed to sense faster moving currents, which are indicative of deeper water, since there is less resistance to fluid motion by rocks/sand and river bed debris. Since fish need deep water to swim upstream and survive, it is important that they tune into this instinct.
Weirs are installed with 3 to 4 gates (indicated by the white squares in the water) that can be opened to allow fish to go through. As shown in the above photo, gates tend to be located near the center of the river, to allow maximum channeling of the water through the weir. Since rivers tend to be deeper in the middles, this design allows fish to swim in the deepest possible water.
When closed, the gates disturb water flow, and thus have a uniform flow velocity as the rest of the weir. However, when you open a gate, there is and open pathway for water to move through. Unrestricted, water is allowed to accelerate from its initial velocity flowing against a restricting barrier to a faster pace, without limiting barriers. This increase acceleration with the same mass, by the equation F=ma will have a greater force. Fish can detect this force, and because they sense a streamlined water column, will swim through the open weir, and continue upstream.
The brain of a fish is designed to sense faster moving currents, which are indicative of deeper water, since there is less resistance to fluid motion by rocks/sand and river bed debris. Since fish need deep water to swim upstream and survive, it is important that they tune into this instinct.
Weirs are installed with 3 to 4 gates (indicated by the white squares in the water) that can be opened to allow fish to go through. As shown in the above photo, gates tend to be located near the center of the river, to allow maximum channeling of the water through the weir. Since rivers tend to be deeper in the middles, this design allows fish to swim in the deepest possible water.
When closed, the gates disturb water flow, and thus have a uniform flow velocity as the rest of the weir. However, when you open a gate, there is and open pathway for water to move through. Unrestricted, water is allowed to accelerate from its initial velocity flowing against a restricting barrier to a faster pace, without limiting barriers. This increase acceleration with the same mass, by the equation F=ma will have a greater force. Fish can detect this force, and because they sense a streamlined water column, will swim through the open weir, and continue upstream.