Changing Habitat Conditions for Fish and Wildlife

Just as the changes that occur to ecosystems vary greatly from project to project, so do changes in habitat. Indeed, painting a picture of all or even most hydroelectric projects as having more or less the same impacts is a serious mistake. For a given project, learning what habitat conditions exist and the extent of ongoing impacts requires a good deal of investigation. Likely sources of information include the project owner, information provided to the Federal Energy Regulatory Commission, state and federal fish and wildlife agencies, and local environmental groups.

When ecosystem changes occur at a project, a new pattern of biological activity and equilibrium is likely to emerge. As this happens, a new and dynamic equilibrium takes hold. With this new equilibrium comes changes to the plants, fish, and wildlife that populate these areas.

Over time, observation indicates which species continue to do well, which ones become attracted or more attracted to the area around a hydroelectric project, and which species begin a gradual, sharp, or complete decline.

To examine these possible habitat changes, let’s begin with fish.


Salmon are the most well-known species of fish that migrate up and down streams. As a species, salmon are called anadromous because they migrate from fresh water to the oceans and then back again. Resident fish, on the other hand, spend their entire lives in fresh water streams, tributaries, and rivers. Some migrate from streams to lakes (adfluvial), others migrate from streams to rivers (fluvial), and some remain in the same reach of water. Brook trout and bull trout, for instance, are well-known resident fish that migrate up and downstream.

Depending on the species, these migration patterns can vary dramatically. In the case of salmon, for instance, coho tend to spawn in small streams and prefer shaded pools with overhanging trees and shrubs; sockeye salmon can migrate hundreds of miles to spawn in large lakes where, as fry, they live for two years before migrating to the sea; and chinook can spawn in large rivers like the mainstem of the Columbia River.

Salmon are called “fry”when they emerge from their spawning area and begin swimming freely and feeding in the stream. As they increase in size and maturity, they become “smolts”or “juveniles.”During their time in the ocean, they become “adults.”As a result, adults only exist in the river when they are migrating upstream to their spawning grounds.

Based on their life cycle and migration and spawning patterns, fish can face a number of different and changing ecosystems. Listed below are the most common and serious fishery impacts that relate to hydroelectric projects.

  1. Slower moving waters in a reservoir can strongly affect salmon for two reasons. First, they can become disoriented in slower moving waters; and second, the length of time it takes smolts to reach the ocean may increase. With disorientation and lengthened travel time comes an increased exposure to predators.
  2. Fish passing through or around a dam can become stressed, injured, disoriented, or die because of contact with turbines, the walls of the dam, or deflection screens. They then exit into a relatively small area where their exposure to predators is increased. While fish passage rates are often better than 90%, fish that must pass through multiple dams face proportionally greater risk.
  3. Supersaturation is a danger for fish going over a dam or through its spillway. If too much nitrogen is absorbed in the bloodstream, air bubbles form and create the equivalent of what divers call “the bends.” At high nitrogen levels, fish and some other aquatic species die. Also, if supersaturation conditions exist, fish passing through or around a dam will absorb greater nitrogen levels and suffer the effects as they continue downstream.
  4. When adult salmon and other fish migrate upstream, the dam can again present itself as a physical barrier. If a “fishway”does not exist, then passage to spawning grounds is lost. While fish ladders are the most common fishways, other examples include fish locks, fish elevators and transportation of fish upstream via truck. Where ladders are used as fishways, fish can find it difficult to find them if sufficient attraction flows are not provided at their base. Once up the ladder, they can again become disoriented and be sucked back over a dam or through its spillway. Salmon do not feed during their migratory journey back to their spawning grounds, so loss of energy and time become critical survival issues.
  5. Reservoirs can create changes in downstream habitat conditions. For instance, macroscopic plants in the Columbia’s estuary are an important food source for salmon migrating to the sea. These plants, however, are no longer as prevalent in the estuary as they once were. Part of this decline has been traced back to conditions caused by reservoirs in the mainstem of the Columbia and on the Snake river.

There can also be effects to fish from loss of riparian vegetation, sedimentation, erosion, and temperature changes. Unlike the impacts listed above, however, these effects are also caused by non-dam activities such as farming, logging, and land development. As a result, when studying the health of habitat along a particular reach of river or tributary, all sources of environmental impacts must be reviewed.

Further, while fish migrating down and upstream may encounter altered ecosystems and barriers that impact their ability to survive, predation from other species also has an impact. Squawfish, for instance, live below a dam where (as predators) they can easily feed on smolts as they come through the tailrace of the dam’s powerhouse.

Likewise, slower moving waters and temperature changes caused by reservoirs can provide improved environments for warm water fish such as smallmouth bass and walleye. These resident fish also prey on salmon smolts moving downstream. And as smolts enter the ocean, the increased presence of sea lions makes survival more difficult.

The introduction of non-native fish to Northwest rivers further complicates the situation. Warm water fish such as smallmouth bass and walleye are examples of non-native species introduced to Northwest rivers by humans. Examples of non-native trout include brook and rainbow trout. While many anglers enjoy catching these fish, it is important to note that their improved conditions are at the cost of poorer conditions for native stocks of salmon and trout.


Riparian vegetation and its bordering waters provide critical habitat for birds, waterfowl, and small and large mammals. When a hydroelectric project results in inundation of a free-flowing river, the nesting, forage, and cover provided by these areas is temporarily or permanently lost.

When habitat is lost, animals are forced to move to higher ground or other areas where habitat conditions may be less suitable, predators are more abundant, or the territory is already occupied. As an example, ground birds like pheasant and grouse require cover and cannot successfully move to higher, more open, ground.

In cases where water levels stabilize at a new height, vegetation in riparian zones can re-emerge and species can re-populate an area. With storage projects, the riparian zone that re-emerges has conditions that now reflect that of a reservoir or lake rather than a free-flowing river. When such conditions occur, certain species will begin to decline, others will become more abundant, and some will populate these areas for the first time.

Ducks and geese are examples of waterfowl that are strongly attracted to the habitat conditions found in reservoirs. For some of these species, reservoirs are providing an important alternative to the wetland areas that they formerly occupied. Canada geese are one example of birds that now frequent reservoirs as part of their migration pattern.