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Effects of the Columbia River dams on salmon population Research Paper

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Updated: May 30th, 2019


The building of dams in the Pacific Northwest started later in the 19th century. These early dams were mainly constructed on small rivers or tributaries for the purpose of irrigation. In the first years of the twentieth century, the first hydropower dams were constructed on the tributaries of the Columbia River (National Research Council 231 ). As the century progressed on, dam building activities were stepped up due to the increase in the demand for hydropower.

The construction of the Bonneville and the Grand Coulee were initiated in late in the 1930s and this was followed by a marked increase in the number of dams and their storage volumes in Oregon, Washington and Idaho (Bonneville Power Adminstration 3).

“Forty five years after the authorization of the Bonneville (1933), 14 dams on the Columbia River and 13 on the Snake River had been constructed” (National Research Council 131). By early 1980s, there were no more sites for large dam construction. Furthermore, public approval for such projects had significantly diminished and thus signaling the end of the growth phase.

At the completion of the rapid growth phase it was now obvious that the dams were posing a major threat to ecological sustainability. Most of the “dams were built on the migratory routes of the most west Pacific Northwest salmon runs” (Stewart 45). Various mitigation measures have been taken to address this issue but not much has been achieved.

This paper therefore seeks to identify the effects of the Columbia River dams on salmon runs. Though the effects are mostly negative, the paper will go further to identify if there is any benefits that have been created by the dams to the salmon population.

Negative effects

Many species of salmon breed and grow in the riffles of cold-flowing rivers far from the sea (Stewart 57). The young salmons often migrate to the sea where they live for several years before returning home to breed. The man made dams have prevented the fish from travelling up and down the river ways, despite mitigating measures such as the construction of fish ladders (Dube 13).

The insufficiency or lack of fish passage facilities have led to the loss of the upstream habitat. The dams constructed across the Columbia River blocks close to one third of the watershed from being accessed by anadromous fishes. The barriers are mainly associated with the Grand Coulee Dam which accounts for a third of the passage barriers (National Research Council 132 ).

The “impassable dams have lead to a significant reduction in the rearing and spawning habitat. However, it is difficult to estimate the magnitude of the loss as record keeping was nonexistent before the dams were constructed” (Pitzer 10). The following are the specific effects of the dams on salmon populations:

Dam-linked deaths

There dams have been found to account for a considerable number of salmon deaths. Even when the dams are built with fish ladders for upstream passage of salmon, “the fish can still be delayed” (Bonneville Power Adminstration 4, par. 2). The rapid flow of water originating from the turbines usually makes it difficult for the salmon fish to locate the tiny attraction flows that guide them to the ladders. There has been a significant improvement in the ladder design and quality of materials used since the 1900s.

However, not much has been achieved in preventing adult and juvenile salmon deaths that can be attributed to difficulties in locating or using the ladder. Some ladders tend to have high flow rates that often and thus are avoided by the salmons causing delays in upstream migration.

The ladder delays may not directly result in the death of the salmons but may interfere with other processes that increase the chances of death. For instance, “salmons do not feed on upstream migration and must use stored energy as efficiently as possible to migrate upstream, mature sexually and spawn successfully” (National Research Council 234, par. 3 ).

Adult salmons usually get killed if they are pulled back into the turbine intakes, although it is difficult to estimate the deaths that result this way. Counting of the salmons between successive dams shows declining numbers that are thought to result from deaths. However, “poaching might account for some salmon losses” (Dube 45). The losses between dams have been approximated to be as high as 25% between Bonneville Dam to John Day Dam.

The dams also pose a significant risk to the downstream passage of juveniles. The juveniles which are often referred to as the guided fish, “can make contact with deflection screen surfaces, gate well walls, the vertical barrier screens in the gate wells, the orifice entrance or portions of the bypass channel or down well” (Stewart 60, par. 4). Such encounters cause a considerable damage to the scale surface and thus significantly reduce the viability of the salmon fish.

The fish undergoes stress when passing through the bypasses. Sometimes the fish is made to “hold in the currents to resist passing downstream” (Dube 46, par. 1). This experience results into physiological stress and can recover if they are held for 48 hours. However, when the fish is under stressful conditions and directly goes through the bypass it often ends up in the outfall, where it may be preyed upon by birds and other fish.

The bypass systems may also result in the concentration of smolts in a relatively small area. Smolts from the large width of the Columbia River usually gather in the narrow bypass channel. “Sometimes thousands of smolts per hour are delivered in a small volume of water to the dam tailrace, which provides a concentrated stream of prey for predators” (National Research Council 236, par. 5 ). Studies conducted previously indicate that there is a high concentration of predators at the areas between the bypass and the outfall.

It is not easy to identify the total bypass mortality due to absence of carcasses. Some researchers investigators consider bypass-caused deaths to include only those which can be observe in the raceways and sampling facilities incorporated in the bypass systems (Dube 56).

But this does not give information on impingement on deflection screens, predation within the gate-well and bypass system, predation caused by bypass concentrated stream of prey, stress-related deaths that occur after smolts leave the outfall are or predation on stressed fish long after they leave the outfall pipe (Pitzer 105).

Time of travel

The time of travel effect is less clear as compared to the effect of passing through the dams. “Ocean type chinook passes their first winter of life at sea while the stream type spend their first winter of life in the stream before going to sea” (O’Laughlin 3, par. 1).

Data revealed by some studies show that sub-yearlings travel downstream faster when the flow rate is high. Sub-yearling Chinook usually grow as they gradually travel downstream in what can be regarded as a “rearing migration” (Bonneville Power Adminstration 4, par. 3).

The yearling Chinook and the steelhead travel faster with an average downstream passage of up to 20 miles or more a day (Stewart 78). The fish passage Center and the Columbia Basin Fish and Wildlife Authority show that dams can result into faster rates with associated implications (O’Laughlin 56).

Studies have been done to identify the survival of the salmons during the downstream passage. A study conducted in 1999 with using a passive integrated transporter (PIT) tags indicate survival near 100% through lower granite pool. However, the estimated mortality across the concrete was higher than expected (Bonneville Power Adminstration 3).

Loss of migratory path

When a diversion is made from a river so that the water is to be utilized for agricultural or domestic needs, there is a high possibility that the downstream migrating salmon and other fish will be drawn into the diversion channel or pipe (Stewart 107). The Columbia River has several channels adjacent to dams and that are not screened adequately.

These unscreened diversions have been present for years despite efforts by the authorities to add new screens and revive the old ones. No study has adequately established the overall benefit provided by the screens in comparison to the lost rearing habitat in irrigation canals and ditches downstream from the screens (Dube 63).

Estuarine dynamics

The estuaries of the dammed Columbia River are thought to have changed and thus affecting the salmon in one way or another. The reservoir storage in the “upper Columbia and Snake rivers have altered both the seasonal pattern and the characteristics of the extremes of fresh water entering the estuary” (National Research Council 236, par. 5 ). The average sediment supply to the estuary has greatly declined and thus affecting the natural salmon habitat.

Effects on spawning habitats

The “anadromous salmon normally bury their eggs in redds in the gravel substrate at varying depths depending on the species” (National Research Council 237, par.4). The presence of large dams prevents the downstream flow of the sediments and this inevitably leads to the destruction of salmon spawning habitats that are located downstream (Bonneville Power Adminstration 5).

The usual practice of regulating the water level in the dams to prevent power surges or increase the provision of electricity often leads to erosion downstream habitats. The resulting “cyclic floods contribute to the extinction of the salmons by flushing away their spawning gravels” (O’Laughlin 3, par. 3).

Urbanization effects

The development of the dams inevitably led to the urbanization of the areas surrounding the Columbia River. The “primary impact of urbanization is the degradation of the downstream water quality through pollution and reduced water flow through removal of irrigation” (O’Laughlin 3, par. 3). Urbanization has led to the loss of forests and thus significantly contributing to the alteration of the temperatures and sedimentation patterns.

Changing temperatures

Temperatures are observed to change significantly when a river is dammed. Normal rivers often have homogenous temperatures due to the constant flow of water. Reservoirs are layered and are worm at the top and cold at the bottom (Stewart 80). When the water is released from the dam it results into unusual cold temperatures which may compromise the survival of the salmon.

Mitigation efforts

The development of dams along the Columbia River was basically for irrigation, protection against floods, production of hydropower, recreation and for navigation purposes. Hydropower production was the main consideration until 1980 when the Northwest power Act was enacted (National Research Council 239).

One of the key areas agreed upon and captured in the act was the protection and enhancement of fishery. Thus profound changes in the operating strategies were to be planned and implemented. This section discusses the mitigation measures that have been taken so far to improve the conditions. This measures “include; fish-passage facilities, predator control, transportation, spill, flow augmentation, reservoir drawdown and dam removal” (National Research Council 239, par. 2 ).

Fish passage facilities

Initially, there were no fish ladders to enable the salmon to swim upstream past the dams. The ones that were constructed in the early 1900s were also found to be inadequate for that purpose. Intensive studies about fish behavior, response to attraction flows, and in ladder hydraulics led to marked improvement in ladder designs (Dube 67). However, the improved designs have not been able to offer a total solution as there are still instances of fishes being delayed by hydropower dams and falling after passage (Dube 68).

The early mainstem dams (rock island dam and the Bonneville Dam) that were constructed on the Columbia River had fish ways that only permitted the passage of adult salmons. All the other mainstem dams that were constructed later also have fish ways. The juvenile passage facilities on most mainstem dams in the Columbia River system use deflection screens that project downward into the intakes of turbines and deflect fish upward from the turbine intake into the gatewell (National Research Council 56 ).

The design of the deflection screen is founded on the fact that downstream migrating juvenile salmon tend to pass through the turbine intake high in the water column (Stewart 83). The screens often guide intercepted fish upward into the gatewell.

Predator control

The major threat downstream migrating juveniles are the squawfish that is alleged to kill millions of juveniles annually (Pitzer 34). A study conducted using the John Day pool revealed that up to 12% of the salmons that entered the pool were killed every month by the squawfish. As a result initiatives were taken to reduce squawfish related salmon deaths. The “programs include bounties and intensive efforts to reduce predator densities” (Stewart 84). It was particularly designed to down size the population of the larger squawfish.


Transport systems have been developed to deliver juveniles downstream. Raceways are normally used to hold downstream migrants for delivery by barges or trucks (Dube 35). The concept is based on “leap frogging” in which the juveniles are delivered at some point downstream from the power projects (Pitzer 87). In the Columbia River system, raceways are used to transport juveniles to the Bonneville Dam tailrace.

This transportation relies on barges which are further used to maintain water quality, for instance, by inhibiting gas super-saturation (National Research Council 243). However, a study conducted on the Snake River showed a reduction of salmon runs when barging was applied which has caused the efficiency of barging to be questioned (O’Laughlin 4).

Flow augmentation

The hydropower dams result in faster passage of the juvenile salmons downstream and therefore significantly reducing their survival rates. This more observed on the middle Columbia and the Snake rivers during spring. An attempt was made to reduce passage through the use of “water budget” which provided some upstream storage in the snake and mainstem Columbia to increase spring flows (Dube 43).


Spill is water that is deliberately passed over the dam to aid fish passage. The river is often spilled for a short time to prevent the accumulation of water bubbles. Spilling is often done in spring and summer during juvenile migration (Dube 53).

Designing of fish friendly turbines

In real sense a negligible amount of fish pass through the turbines as compared to other passages (Pitzer 45). However, dam operators are slowly removing the old turbines and installing new ones that are designed for fish safety.


This paper sought to identify the effects imparted by dams on Columbia River Salmon populations. The investigation was supposed to identify both the positive and negative effects.

Several negative effects have been identified and they include; increase in mortality rates, loss of habitat, increased time of travel, change in estuarine dynamics, loss of migratory path and finally effects associated with industrialization.

All the literature reviewed had no data on the positive effects of dams on salmon populations. It remains to be investigated whether there are any positive effects impacted by the dams on the salmon populations.

The paper has also identified the mitigation measures that have been taken to improve the viability of the salmon population in the Columbia River. The following measures are currently being undertaken; Predator control, transportation, flow augmentation, spill, designing of fish friendly turbines, fish passage facilities (National Research Council 142).

Works Cited

Bonneville Power Adminstration. “Managing the Columbia River System helps fish .” Govtcorp.com. 4 September 2010. Web.

Dube, Kathy. The effects of large dams on salmon spawning habitat in the pacific northwest . Seattle: Waterdhed Geodynamics, 2003. Print.

National Research Council . Upstream: salmon and society in the Pacific Northwest. New York : National Academies press, 1996. Print.

O’Laughlin, Jay. The varriable impacts of dams on Columbia and Snake river salmon populations. Moscow: University of Idaho, 2001. Print.

Pitzer, Paul. Grand Coulee: Harnessing a Dream. Washington: Washington state University , 1994. Print.

Stewart, Holbrook. The Columbia: The Classic Portrait of the Great River of the Northwest. New york: Comstock Editions, 1986. Print.

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