Dams and Salmon: Struggling to Survive in the Snake River

There are four federal dams on the Lower Snake River - Ice Harbor, Lower Monumental, Little Goose and Lower Granite. They were built from the 1960's to 1980's to generate energy, provide water for irrigation, and simplify barge transportation of goods down the river. But their impact isn't just economic. Dam construction created a ripple of changes that continue to negatively affect wildlife in the river ecosystem. In this StoryMap, we'll dive into the impacts on four species of salmonids and the river's departure from its natural rhythm.

The 1,078 miles of the Snake River flow through Wyoming, Idaho, and Washington state, making it the  ninth longest river in the US . The Army Corps of Engineers operates fours dams in the Lower Snake River that control the flow of 140 miles of river. This creates a large reservoir upstream of each dam and narrows the river downstream. As hydroelectric power plants, they contribute to renewable energy production by producing over  1,000 megawatts annually .

Salmonids are a family of ray-finned fish. The Oncorhynchus genus of Pacific salmon and trout is named after the characteristic hooked lower jaw in spawning males.

1. Sockeye (Oncorhynchus nerka): notable for the vibrant red color that males develop during spawning season.
2. Steelhead (O. mykiss): known for their superior swimming speed and jumping ability.
3. Chinook (O. tshawytscha): have several different migration patterns (spring, summer and fall Chinook).
4. Coho (O. kisutch): were declared functionally extinct in the Snake River before being reintroduced.

Sockeye, steelhead, and Chinook salmon are all protected under the Endangered Species Act due to significant population decline. Coho were previously declared functionally extinct in the Snake River, but a hatchery release program implemented by the Nez Perce Tribe brought the fish back to the river.

All of these fish species are anadromous, which describes their life history of emerging from eggs in freshwater, traveling to the Pacific Ocean and living for a year or more in saltwater, and then returning to their freshwater birthplaces to spawn. The existence of dams along their migratory pathways impedes their journey down and up the river.

The physical structure of a dam negatively affects natural flow conditions and blocks or limits fish passage in both directions. But there are also environmental factors, like changes to water temperature and riverbed composition that affect the core of the salmonid lifecycle.

How do seasonal temperature patterns of the river impact salmonid survival and spawning?

Salmonids need cold water to thrive and  adults are sensitive to summertime temperatures higher than 68 °F  (orange on the graph). High temperatures can decrease spawning ability and reduce swimming speed. The stress of heat can be lethal and warm water makes fungal infections and other disease more common.

To avoid the risky warmwater, salmonids may look for cold water refuges in tributaries of the main river. Swimming into a tributary diverts fish from their main objective of traveling upstream to spawn.

In free-flowing conditions, the temperature has more variation than when dams control water flow. The large reservoirs behind dams heat up more quickly than a normal river would.

While free-flowing rivers may have a few uncomfortably hot days for salmonids,  dams stretch out the number of days with risky river temperatures . The large orange block from July to September shows how fish must swim through hot water at all four Snake River dams.

Even if warm water accumulating in the reservoirs doesn't kill a salmonid, it may still cause stress or discourage the fish from continuing to migrate.

In response to high water temperatures, cold water releases are scheduled for summer months. The cold water sinks to the bottom of the reservoir, while warm water rises to the top.  Some fish will linger in the layer of cold water and delay migration, reluctant to risk swimming into the warmer water .

As global temperatures rise due to climate change, there are concerns about how this will affect cold-water dependent species like salmonids. How will anadromous migration change if fish must pass through warmer and warmer waters?

In 2015, a severe heat wave resulted in high levels of fish mortality; 99% of the Snake River sockeye salmon who successfully passed Bonneville Dam on the Columbia River died while traveling upstream to their spawning grounds.

How does riverbed composition affect spawning?

Substrate quality is key to the spawning process. The Snake River has an armored riverbed where coarser gravel lies on top of finer sediment.

Female salmonids  construct nests , called redds, by hollowing out an area of gravel with their tail fins, laying eggs in the hollow, and covering the eggs with rocks. Different fish species have specific preferences for gravel size - for example,  Chinook prefer gravel between one and six inches in diamter . The water speed and depth also play a role in finding the perfect site. Dams make it challenging for female fish to find an ideal spot for a redd.

Dam construction eliminated  42% of main-stem river habitat  in the Snake River. Access to some river habitat far upstream has been blocked completely.

There are  many factors  that impact egg survival. After the eggs have been hidden with gravel, it's important that oxygenated water can still flow in and out of the redds. It's essental that clean, oxygen-rich water nourishes the eggs during this vulnerable stage. If the gravel gets clogged with fine sediment, blocking the flow of water, the eggs will be less likely to hatch.

Because it's so difficult to find an ideal site,  redds are often made in colder environments . In colder water, eggs hatch later, which disrupts the timing of migration.

When the eggs hatch young alevins emerge. They still have a yolk sac attached for nutrients, and hide in the gravel as they grow. When their yolk sac runs out, they explore the streambed for invertebrate meals.

By flooding shallow edges of the river upstream of the dam, dams make river edges steeper. The loss of shallow waters  takes away critical habitat  for young fish. Shallows provide shelter from predators, an abundance of invertebrate food, and warm temperatures to boost growth.

A unique feature of steelhead is their ability to spawn more than once in their lifetime, or iteroparity. Iteroparous steelhead can go back to the ocean after spawning and swim back upstream again to spawn in a subsequent year, while semelparous salmonids can only spawn once (and typically die a few weeks after spawning).

Steelhead are also unique in their  speedy swimming and jumping ability . This strength allows steelhead to clear physical barriers and spawn further upstream than other salmonids.

Iteroparity rates of Snake River steelhead are the  lowest recorded for the steelhead species . This is important because repeat spawners produce larger eggs and provide additional potential for population growth.

 Factors that could contribute to low iteroparity rates in the Snake River :

• Having to pass several hydroelectric dams.
• Stress from dam passage and high summer temperatures.
• Long and steep migration path (more challenging than the route of other steelhead populations).

How does the flow of water affect riparian plants that grow along the side of the river?

Riverside, or riparian, vegetation are wild plants that grow along the water's edge. These plants have the special ability to adapt to both flooded and dry conditions throughout the year. Exotic plants are generally less successful than native plants at adapting to variation in river levels.

Un-dammed rivers have a tempermental nature and will flood the riparian zone unexpectedly. When dams are constructed, they  block the natural flow of the river  and create an artificial controlled flow environment. Without the natural variation of the river, exotic plants have a better opportunity of surviving and potentially crowding out native plants.

To combat this effect, some dams are using controlled water releases to simulate natural flows. Current research is investigating how to optimize this strategy.

How does the strategy of releasing fish raised in captivity into the wild impact migration patterns and spawning?

To increase populations, hatcheries can release young fish into freshwater streams in the hopes that they migrate to the ocean and return to spawn. It is difficult to recreate wild genetic variation in a hatchery program, although the original stock are typically selected to be as close as possible.  Rapid genetic adaptations  cause hatchery-raised salmon to have a lower reproduction rate than wild salmon.

When fish return to spawn, they aren't all equal. Some returning male fish will exhibit a "minijack" phenotype, meaning that they are too small and did not spend enough time growing in the ocean. This is a  negative outcome  that may skew the sex ratio, alter genetic balances, and even impact migration patterns.

In a study comparing groups of wild and hatchery-raised fall Chinook, the hatchery fish were more than two times more likely to return as minijacks than the wild fish.

Consideration of the impact of hatchery fish on wild fish is important, and many hatcheries have practices that try to offset the spawning timing between wild and hatchery fish to prevent mixing.

Coho salmon were abundant in the Snake River basin before construction of the Lewiston Dam in 1927 restricted their access to Idaho spawning habitat in the Clearwater River. It is estimated that  200,000 coho returned to spawn  each year in the early 1900's.

In 1985, following several years of zero coho returns logged, the Snake River population of coho was declared extirpated. In 1994, the  Nez Perce Tribe began recovery work to reintroduce coho to the Snake River . Using eggs recovered from the Lower Columbia River, they collaborated with hatcheries to release juvenile salmon into small tributaries of the Clearwater River. As generations of coho returned to spawn, some were integated into the hatchery stock.

While the program successfully reintroduced coho to the river, the population is not self-sustaining. Coho salmon must navigate eight federal dams to reach the ocean. Most released juveniles perish on their downstream passage, with  estimates ranging from 50% to 70% . The Tribe continues to release large numbers of coho smolts annually to maintain the population level.  In 2021, 1.5 million smolts were released for a return of 24,000 adult coho, a return of only 2% .

The Nez Perce Tribe has worked with hatcheries for decades to restore coho, but they continue to advocate for the removal of the four Lower Snake River Dams to allow fish populations to be self-sustainable without the assistance of hatchery releases.

Fish ladders provide a pathway for adult fish to travel upstream. They are not useful for juvenile fish migrating downstream. All four Lower Snake River dams were constructed with fish ladders to allow adult passage.

Safe juvenile passage is especially important, and  strategies to increase juvenile survival  are specific to their size and behavioral characteristics.

By 2009, the four Lower Snake River dams were all  retrofitted with spillway weirs . The concept was to enable downstream passage for juvenile fish swimming near the surface. Without these systems, juvenile fish had to dive 50 to 60 feet into the reservoir to look for passage entrances. There are two types of spillway weirs:

• Removable Spillway Weirs: Allow fish to go in a "waterslide" through the center of the dam.
• Temporary Spillway Weirs: Fish are directed to a "waterfall" that spills them over the dam.

Fish bypass systems redirect fish away from turbines and into a safer passageway. Some dams collect fish in a truck and drive the fish downstream for release.

Changes to turbine structure can make turbines less lethal for fish. Despite new designs, turbines are still considered dangerous for fish. Ice Harbor Dam is installing new turbines in an effort to increase survival of juveniles who do not enter the bypass system or spillway passage routes.

Despite ongoing adaptations to the dam structure, all four salmonid species remain at concerningly low levels. This implies that these changes alone are not sufficient to restore self-sustaining natural populations.

Complete removal of the physical barrier preventing fish from migrating is the most simple solution from the perspective of threatened fish. While fish ladders, spillway weirs, cold water releases, and hatchery programs can all increase fish survival, these programs are only necessary because of the existence of the dams.

The decision to remove Condit Dam was based on a variety of factors, but the lack of fish passage played a large role. Without fish ladders to swim up, spawning grounds upstream of the dam were completely blocked to fish. The dam was almost 100 years old, and  it was too expensive  to install fish ladders and update operations to be in compliance with federal requirements.

During the removal process, engineers chose to  blast a hole  into the bottom of Condit Dam with dynamite, explosively releasing a huge rush of sediment and water. The sediment buried plants and aquatic organisms, but flows have normalized over time.

Post Dam Removal Wins

• Spawning habitat increased by 46% for fall Chinook that historically used this river for spawning.
• Steelhead and coho have been observed in upstream tributaries of the river.

From a natural resources perspective, dams represent a major obstacle to anadromous fish survival in the Snake River. From a political perspective, the requirement for federal approval of dam removal poses a significant obstacle to breaching the dams. However, the cause in gaining attention and traction in the federal government.

In late April 2022, the House of Representatives Natural Resources Committee  approved a bill to study Pacific salmon  and make management recommendations in honor of late Republican Representative Don Young.

In July 2022, the Biden Administration released a statement featuring a  new federal report  that recommends removal of all four Lower Snake River dams as a necessary step to restore Snake River salmonid populaions. The draft report, written by scientists at the National Oceanic and Atmospheric Administration, emphasizes the importance of large-scale actions and warns that climate change will increasingly complicate future efforts to restore these fisheries. The White House press release also included a  report contracted by the Bonneville Power Administration  which quantifies concerns about the feasibility and coast of removing the dams and sustainably replacing the hydroelectric energy they generate. While acknowledging that currently "business is usual" is insufficient for salmonid recover, the chair of the White House Council on Environmental Quality has yet to indicate the official Biden Administration stance on dam removal.

Looking ahead, these reports represent a federal interest in this topic and ripples of hope for the potential recovery of Snake River salmonid populations in the long term.

All terms are defined in the context of their use in this StoryMap. Information was compiled from a variety of sources.

• Alevin: A newly hatched juvenile fish that still has a yolk-sac attached for nutrients.
• Anadramous: A fish characterized by migrating from freshwater to seawater to grow, and returning to freshwater to spawn.
• Exotic (plants): A plant species that is not native to the area but can still compete for resources and space.
• Extirpation: Local extinction, where a population is eliminated from a specific areas but still exists elsewhere.
• Hatchery: An operation that produces eggs, raises juvenile fish, and releases them into the wild.
• Iteroparous: Reproducing more than once per lifetime.
• Minijack: A male fish that returns to spawn at a smaller size than expected.
• Phenotype: The physical appearance of an organism.
• Redd: A salmon nest formed by burying eggs in gravel.
• Riparian (ecosystem): The area along the shores of a river.
• Semelparous: Reproducing once per lifetime.
• Smolt: A juvenile fish that is mature enough to begin migration to the ocean.
• Smoltification: The maturation process of adapting from freshwater to seawater.
• Substrate: The bottom surface of the river, including rocks and soil.
• Tributary: A small stream or river that flows into the larger, main-stem river.