What is a Fish Ladder and Weir?
A fish ladder (or fishway, fish pass) is a structure designed to allow fish the opportunity to migrate upstream over or through a barrier to fish movement. The very best fish passage is the complete removal of the barrier. When this is not possible, fishways can be installed to allow fish passage over or around the barrier. Fish ladders may be recommended when blocking structures are as low as 1 to 2 feet in height. Critical factors evaluated to determine if a ladder is necessary include the water depth below the blockage, the height of the barrier, the water velocity over or through the barrier, the quantity and quality of fish habitat upstream of the barrier, fish movement patterns (i.e., are fish being blocked from moving upstream), and the species composition of the fish community (i.e., which species need to pass).
Why Do We Need Fish Ladders?
It was recognized long ago that obstructions in rivers such as dams fragment aquatic ecosystems and affect fish populations. Michigan's rivers are fragmented by over 2,500 impassible registered dams and likely twice as many other barriers such as unregistered dams and poorly designed culverts. Fragmentation of rivers can and has resulted in the decline of fish production from those waters and in some cases a complete loss of fish species. Most fish species are affected by fragmentation but species such as steelhead, Chinook salmon, lake sturgeon, or suckers that are required to migrate to spawning grounds are particularly susceptible to declines from impassable river obstructions. Thus, effective fish passage can be critical to the protection and recovery of many fish stocks.
What Factors Need to be Considered When Designing a Fishway?
Many design factors are looked at by engineers when designing and placing a structure. Every barrier in a river represents a unique situation and challenge, and each fish ladder is therefore carefully designed and placed. No one fish ladder design will accommodate all species of fish at every location. Each species has different physical characteristics that need to be taken into consideration when designing passage facilities. Trout and salmon have exceptional burst speeds so they can swim through fast water. Species like walleye, northern pike and smallmouth bass lack such burst speeds and require slower water to get around or over barriers. Flows, energy dissipation, resting areas, drop between pools (fishway slope), attraction velocities, entrance locations, and space in pools are just some of the factors that need to be considered when designing a ladder.
Types of Fish Ladders or Fishways
There are five basic designs for fish ladders that are used throughout the world: pool-weir; vertical slot; Denil; steeppass; and natural bypasses or fishways. All five can be found in Michigan.
Pool-Weir Fishway at Berrien Springs Dam
The pool and weir fishway is used at many man-made structures and is the oldest of the fishway designs. Pool and weir fishways all have a set of steps and often have a hole (orifice) in the bottom of each baffle to control the plunging effect and to create another means for fish to move upstream. This design frequently is used at larger barriers. Pool-weir designs work well for jumping species like trout and salmon and are not as effective for species that do not jump, particularly if the ladder does not have an orifice in the bottom. This fishway type is sensitive to water surface elevation changes in the impoundment or upstream river.
Vertical Slot Fishway
Vertical slot ladders are quite common and use a large narrow slot to control water flow and depths in the pools between slots. Vertical slot fishways are more complex than pool and weir ladders, but they have a constant flow pattern at all operating depths and are not sensitive to impoundment or upstream water surface elevation changes. This design allows fish to pass at any depth and is effective for a wide range of species as long as current speed in the baffle is not too fast. This design frequently is used at larger barriers.
Vertical Slot Fishway at Niles Dam
Denil Fishway at Trout Creek Dam
Denil fish ladders use what is often described as a roughened ramp approach, which is a different approach from one using pools with weirs or vertical slots to pass fish around barriers. The ladder has a large number of baffles that act like a set of rapids with a wide range of water speeds so (if properly designed) many fish species can successfully use this type of fishway. This fishway also can handle a wide range of changes in the upstream water surface elevation either in the impoundment or river. You can see this design at either small or large barriers.
Steeppass fishways use the same principle as Denil fishways and use many baffles to simulate a rapids to a fish and present the fish with many options with respect to water speeds. This type of fishway, when properly installed, can pass a wide range of fish around barriers. These are typically pre-fabricated out of metal in 10 foot sections and are usually narrower than Denil fishways. They work best at small barriers where inexpensive passage is needed. This fishway can also handle a wide range of changes in the upstream water surface elevation either in the impoundment or river.
Steeppass fishway at Little Platte Lake Dam
Steeppass fishway in Alaska
Natural Bypasses or Fishways
Natural Bypass Fishway at Williamston Dam
This design looks much like a natural stream and has the ability to move all species when designed properly. A stream channel using natural materials is designed to bypass a barrier. It can replace lost stream rapids areas from impoundment, can provide replacement stream habitat, and can be used on any sized river but does require the most space to properly site it. They also can provide canoeing or boating access around a barrier.