“Good leaps depend on the pool below the falls. The deeper the pool, the better the take-off angle, and the faster the fish can go. Consequently, “the higher the fish can leap,” says education coordinator Jonathan Lyman of the Alaska Department of Fish and Game.
Also, water falling from a ledge above hits the water surface below, and continues to plunge deep below the surface. Then — under pressure from more plummeting water — the water rushes back up to the surface. The continuing falling water acts as a hydraulic jack, squeezing the water back up to the surface. Fish use the “hydraulic jump” water to boost their initial leap, says Webb.
Among Pacific salmon, Lyman says, Coho and steelhead (rainbow) trout are the best leapers because they seek the high river source to spawn. The chum salmon is the poorest leaper. The Atlantic salmon is the best leaper of all salmon.” (source)
Water depth of the pool below the falls and the hydraulic push of the plunging water back upward combine to propel the determined fish.