Fish passage

Figure 1. A fish ladder at a hydraulic dam.

A fish passage is any device used to promote and regulate safe fish migration up and downstream from hydroelectric facilities. If fish cannot migrate and rear without interruption, populations can significantly decrease. Fish passages help to prevent harm from coming to river ecosystems by providing fish and other forms of aquatic life safe methods of passing through hydroelectric facilities. Types of fish passage include fish ladders, fish lifts, and juvenile bypass systems.

Although fish can bypass dams by taking the path that water flows through the turbines, the spinning blades generally cause death or injury. Fish may also undergo stress and disorientation when pushed through the high velocity passage, which makes them susceptible to predators at the end of the passageway. Although turbine designs have been improved through the use of rounded parts, there are still significant risks for fish to move through turbines.

Types of Fish Passages

There are a few main types of fish passages that allow fish to move up or downstream. They are outlined below.

Fish Ladder

A fish ladder is a structure that allows fish to climb to a higher elevation. There are several different specific designs of fish ladders, but the overall construction remains the same. Fish ladders are made of a series of slopes and pools that resemble a flight of stairs. This long, slightly sloped (generally they have no more than a 10% incline) stairway includes a series of pools known as weirs. These weirs allow fish to have an area to rest as they climb the ladder without being pushed downstream. The goal of these is to prevent fish from experiencing too much stress when climbing the ladder.

To attract migrating fish to the ladder, it must accurately simulate downstream flow. By simulating downstream flow, fish will naturally swim against this flow and climb the ladder. This natural flow and slight slope allow fish to move upstream and uphill to pass the dam safely, not being diverted through the dam itself.

Fish ladders pose issues for weaker or non-jumping species, as they are generally designed for stronger swimming fish or jumping fish such as trout and salmon.

Fish Lift

Fish elevators and locks are collectively known as fish lifts. Fish lifts are used in certain special situations because they are not species specific, nor are they flow dependent like fish ladders are. These lifts attract fish to a chamber filled with water at the downstream side of the hydroelectric facility and transport them beyond it.

Similar to fish ladders, the lifts attract fish into a hopper or chamber with the use of flowing water, which guides the fish into the elevator. After a set amount of time or if a sufficient amount of fish has been detected in the chamber, the elevator will carry the contents up to the forebay level and release the fish onto the upstream side of the dam.

Fish lifts have several benefits over the use of fish ladder technologies. Specifically, fish lifts tend to work better when large numbers of fish are migrating, or the migrating fish cannot jump well and move up the latter. However, there are some downsides to fish lifts. For example, crowding can occur in peak migratory periods. Although the usage of fish lifts may accommodate for a larger quantity and variety of species, fish lifts are more prone to failure and thus can lead to a higher operating and maintenance cost as well as time lost due to malfunctioning or maintenance.

Juvenile Bypass Systems

This is a special system that allows juvenile fish to move downstream from hydroelectric dams while completely avoiding the turbine passage, which could otherwise cause death or injury. This system channels young or smaller fish into a special gatewell through the use of a fish screen. This gatewell is simply a small, upwards chute of water. From the gatewell, the small fish are collected and transported through a special channel from the turbine into the downstream reservoir.