Several things must be considered when building a natural bypass fishway. First of all, the maximum slope for natural fishways is 1:20 (Schaa et al. 1999, p.68). The height of fall in Åminnefors is 5 meters. Therefore the fishway should be at least 100 meters long. In Billnäs the height of fall is 7 meters and therefore the fishway should be at least 140 meters long (Jormola 2010).
International recommendations state that the flow in a natural bypass fishway should be about 5% of the average discharge. Because the average discharge in the Mustionjoki River is about 20m3/s, the discharge in a fishway should be 1m3/s. In addition, another 5% of the average discharge of the river could be used occasionally to increase the amount of discharge downstream, because the migrating fishes are attracted by relatively strong currents. The fishes follow the strongest current of the river. Becaue the level of the surface of the water as well as other circumstances vary, the fishways must be designed in such way that they work well with variable flow rates (for example from 0,5 m3/s to 1 m3/s) (Jormola, 2010).
The maximum velocity of flow in the fishways is 2m/s and it should be reached only with a few locations because otherwise especially the smaller fishes would not have the strength to swim all the way up. However, in the entrance the velocity of flow should be in its maximum (though never more than 2m/s) to attract the fishes effectively. The entrance should be close to the turbines of the hydro-power plant (because the flow which comes from the turbines attratcs the fishes). (Schaa et al. 1999, p.46-47, 68)
The fishways should be connected to the botoom of the river with a slope whose inclination is no more than 1:20. This ensures that the fishes which swim in the bottom of the river as well as some insects which live in the bottom of the river are able to reach the fishway. The angle of discharge should be no more than 45° (Schaa et al. 1999, p.48). The fishways should be open and if they are built for example inside a building it must be made sure that daylight reaches the channel naturally. Fishes avoid dark channels. (Schaa et al. 1999,p.49)