Fish Ladder - Types

Types

There are five main types of fishways:

• Pool and weir
• Baffle fishway (Denil, Larinier, Alaskan Steeppass, or other baffle configuration)
• Fish elevator
• Rock-ramp fishway
• Vertical-slot fish passage

A pool and weir is one of the oldest styles of fish ladders. It uses a series of small dams and pools of regular length to create a long, sloping channel for fish to travel around the obstruction. The channel acts as a fixed lock to gradually step down the water level; to head upstream, fish must jump over from box to box in the ladder.

A baffle fishway uses a series of symmetrical close-spaced baffles in a channel to redirect the flow of water, allowing fish to swim around the barrier. Baffle fishways need not have resting areas, although pools can be included to provide a resting area or to reduce the velocity of the flow. Such fishways can be built with switchbacks to minimize the space needed for their construction. Baffles come in variety of designs. The original design for a Denil fishway was developed in 1909 by a Belgian scientist, G. Denil; it has since been adjusted and adapted in many ways. The Alaskan Steeppass, for example, is a modular prefabricated Denil-fishway variant originally designed for remote areas of Alaska.

A fish elevator or fish lift, as its name implies, breaks with the ladder design by providing a sort of elevator to carry fish over a barrier. It is well suited to tall barriers. With a fish elevator, fish swim into a collection area at the base of the obstruction. When enough fish accumulate in the collection area, they are nudged into a hopper that carries them into a flume that empties into the river above the barrier.

On the Connecticut River, for example, a fish elevator lifts up to 500 fish at a time, 52 feet (15.85 m), to clear the Holyoke Dam. In its first year of operation, 1955, the Holyoke fish elevator carried 4,899 shad over the dam; by 2004, the typical annual number of fish lifted had risen to more than 500,000.

A rock-ramp fishway uses large rocks and timbers to create pools and small falls that mimic natural structures. Because of the length of the channel needed for the ladder, such structures are most appropriate for relatively short barriers. They have a significant advantage in that they can provide fish spawning habitat.

A vertical-slot fish passage is similar to a pool-and-weir system, except that each "dam" has a narrow slot in it near the channel wall. This allows fish to swim upstream without leaping over an obstacle. Vertical-slot fish passages also tend to handle reasonably well the seasonal fluctuation in water levels on each side of the barrier.

• John Day Dam fish ladder on the Columbia River, North America

• Fish elevator

• Fish ladder in Meuse River the Netherlands

• Fish ladder in North Vancouver, Canada

• Detail of fish ladder in Uppsala, Sweden

• Detail of fish ladder on the River Dart in England.

• At the Charles River Dam near Boston, United States

• Seasonal ladder at the Pico Blanco Scout Reservation on the Little Sur River, United States

• Fish ladder for salmon near the power station in Gullspång, Sweden.