In fishways, the entrance is important for successful fish ascent. Different salmonids have unique requirements for water flow and velocity for optimal entry and passage as well as specific behavior patterns in the fishway. Orsborn (1986) conducted abroad survey among those who had designed and operated fishways. According to the results of this questionnaire, there was a great variety in the successful designs, that were used for species with similar swimming capabilities. No one has actually attempted to determine the main reasons for this. In 1941, in discussion of pool-and-weir fishways, McLeod and Nemenyi stated that the overfall-type has the advantage of being attractive to the fish. Previously several fishways were equipped with an additional entrance tank mostly with overflow. For example, at the Frazer dam, in Alaska, a steeppass fishway is equipped with a U-shaped entrance to form the enclosed tank with a submerged vertical slot opening (adjustable from 22 cm to 30 cm). The construction proved out to be successful for the ascent of sockeye salmon (Blackett 1987). Orsborn (1986) stated that the factors affecting success of the passage opening are difference in pool elevation, width and type of the opening, jet geometry, and flow control. These factors are interdependent.
In the laboratory studies at the University of Oulu, Finland, it was observed that jet geometry below the fishway entrance varies with fishway type and discharge. Also jet dispersion and flow decay varies for different types, as does the distance from the fishway entrance at which the jet is still noticeable. Flow decay below the entrance was studied for pool-and-weir fishways with horizontal and V-shaped weir crests and for vertical slot and Denil fishways. The highest velocities inside the discharging jet differed with fishway type from about 10 to 20 times the mean velocity at the cross section in Denil and pool-and-weir fishways to about 30 to 50 times the mean velocity for pool-and-weir fishway with V-shaped sharp crested weir and up to about 40 to 50 times the mean velocity at the cross section with the highest velocities being as much as 80 times the mean velocity at the cross section for the vertical slot fishway.
The differences in highest velocities in different fishway types are due to the three dimensional character of the flow. Flow from pool-and-weir and Denil fishways is widely spread over the cross-section. The flow from Denil fishway is directed up to the water surface, while the flow from pool-and-weir fishway is directed downwards the bottom. The flow from pool-and-weir fishways with a V-shaped crest and from vertical slot fishways is concentrated into a narrower range over the cross-section.
The results of the studies can be used in determining the most appropriate solution for the fishway entrance at different sites and for different fish species. Because the nature of the flow from different fishway types differs so, special attention should be paid to what the desired flow pattern below the fishway entrance is. Guidelines for this are given by biologists.