Salmon Lifecycle and the Effects of Dams

Dams disrupt the life cycle of salmon.  They often block upstream travel by adults and many juvenile salmon are killed as they make their way downstream.  This is why salmon is a strong contributing factor in the removal of dams in the Northwest.

Chinook salmon migration.
Courtesy of Sooke Salmon Enhancement Society.

Salmon are anadromous fish, meaning they spend part of their life in the saltwater of the ocean and part of their life in the freshwater of rivers.  Salmon are born in freshwater as fry, who spend most of their time hiding from predators.  Fry often spend two to three years growing in the rivers, depending on species, before starting to migrate toward the ocean.  Next, salmon must undergo smolting in order to manage the physical effects of changing from a freshwater environment to a saltwater one.  Salmon enter the ocean as juvenile adults and often spend much of their adult life migrating north to Alaska, to feed and reach sexual maturity.  Once salmon reach sexual maturity, their homing instinct kicks in and they head south to return to their home stream or river to spawn and lay their eggs.  Most salmon die within a week of spawning, leaving their decomposing bodies as an important source of nutrients in the stream.

Pacific Salmon Life Cycle
Courtesy of Capital Regional District
 (http://www.crd.bc.ca/watersheds/protection/wildlife-plants/salmon.htm)

Before dams, the odds that salmon would survive to return to their home stream to spawn was small (see picture).  With the arrival of dams in the early to mid-20th century, the challenges salmon faced for survival only increased.

Dams present difficulties for salmon traveling upstream and downstream.  Juveniles migrating downstream must pass through the turbines of the hydroelectric dams if there is not a bypass system. If the juveniles are forced to pass through the turbines, many are killed by the turbine blades.  One solution to this problem is by spilling water over the spillways at dams, which allows the juveniles to avoid going through the turbines.

Grand Coulee Dam Spillway
Courtesy of Bureau of Reclamation.

There are also high mortality rates for juveniles in reservoirs.  Reservoirs slow migration to the ocean to one to two month, where salmon are genetically programmed to migrate over the course of a few weeks.  Furthermore, the longer migration time and the increase of slow-moving or still water behind dams makes the juveniles more vulnerable to predation.

Salmon have amazing jumping abilities!
Courtesy of NOAA Photo Library.

Grand Coulee Dam
Courtesy of Gregg M. Erickson

Additionally, adult salmon traveling upstream to their spawning grounds are impacted by dams.  Before the dams were built, salmon managed to jump up waterfalls, but they simply cannot jump over dams.  Dams have closed off anadromous fish access to one-third of the Columbia River watershed.  Of particular note is the Grand Coulee Dam on the Columbia River in northeastern Washington, which eliminated all of the spawning grounds upstream of the dam.  At the time of construction in 1933, Washington Fisheries decided that fish passage at giant concrete wall was "impossible."  

Even if the dam provides fish passage options, they still pose many challenges for salmon.  They often have hard time finding the opening to the fish ladder because of fast and turbulent water at the base of a dam.  Furthermore, fish ladders can delay the upstream travel if the flow of water in the ladder is too high or if the adults are sucked back over the dam through the turbines.  While delay is not necessarily fatal, most adult salmon do not eat on their upstream journey, so they must their energy efficiently in order to reach the spawning site and lay eggs.

Fish ladder at the John Day Dam.
Courtesy of the U.S. Army Corp of Engineers.

Ultimately, dams provide many challenges to salmon survival, both at the beginning of the life cycle as they migrate downstream to the ocean and as they return upstream to the spawning grounds at the end of their lives.  On the Columbia River, the Pacific States Marine Fisheries Commission estimates that dams account for the deaths of 70%-96% of juveniles and 40% of upstream-traveling adults.      Currently, many populations of Pacific salmon are listed as endangered or threatened under the Endangered Species Act, which has increased pressure on dam operators to modify or renovate the dams to allow for better fish passage and survival.  In some cases, such as the Marmot Dam on the Sandy River (OR), the operator decided it made better economic sense to remove the dam than attempt to renovate infrastructure from the early 20th-century.  In this sense, it can be said that salmon are starting to bring down the dams.