Adult Fish Returns

Bonneville Dam Adult Fish Returns. Four graphs showing Chinook, Steelhead, Sockeye, and Coho salmon respectively.

Lower Granite Dam Adult Fish Returns. Four graphs showing Chinook, Steelhead, Sockeye, and Coho salmon respectively.

Lamprey statistics. Two graphs. One for Bonneville Dam and one for Lower Granite Dam.

Cooling the Snake River

Lower Snake River Dams: A Value to the Nation

The Snake River is the principal tributary to the Columbia River, draining approximately 107,000 square miles in Washington, Idaho, Oregon, Wyoming, Utah and Nevada. In the 1960s and early 1970s, the federal government built four large dams on the Snake River: Ice Harbor, Lower Monumental, Little Goose, and Lower Granite.

The U.S. Army Corps of Engineers' Walla Walla District owns and operates the four lower Snake River dams, all of which are multiple-use facilities that provide navigation, hydropower, recreation, and fish and wildlife conservation benefits.

Because of their locations, size and ability to help meet peak power loads, these four dams do much more than generate energy--they are key to keeping the system reliable and helping to meet its multiple uses — including supporting wind energy. The Snake River dams lie east of the other federal generators, so they provide a significant technical contribution to transmission grid reliability.

The Lower Snake River system of locks and dams deliver a significant economic benefit to the nation. Barging on the inland Columbia Snake River System moves, on average, approximately 10 million tons of cargo valued at over $3 billion each year. Forty percent of the Nation’s wheat transits through this system.

Fish Passage Improvement Study

Lower Snake River Fish Passage Improvement Study: Dam Breaching Update

Plan of Study 2010

Lower Granite Fish Ladder Temperature Improvement

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Fish cooling systems installed at Lower Granite and Little Goose Dams

The U.S. Army Corps of Engineers has developed and constructed fish cooling systems at Lower Granite and Little Goose dams to alleviate warming water concerns.

Warm water temperatures above 68 degrees aren’t good for salmon and steelhead in the Columbia and Snake river system, and records show that 2015, 2016 and 2017 were the hottest years on record.

When summer temperatures spiked, the Walla Walla District’s scientist, biologists and engineers responded by developing fish cooling systems at Lower Granite Dam and Little Goose Dam on the Snake River.

While building four dams on the lower Snake River between the 1950s and 1980s, the U.S. Army Corps of Engineers constructed adult fish ladders to allow passage of upstream-migrating adult salmon and steelhead (salmonids) on their way to their natal spawning areas. Since construction, the Corps has made both facility and operational improvements to assist adult salmonid migration as needed.

Historically, the Snake River is known to have experienced high summer water temperatures in years prior to construction of dams. In summer 2013, elevated water temperatures began to occur in Columbia-Snake basin river reaches with and without dams due to unusually hot weather dominating the basin. At Lower Granite Lock and Dam’s adult fish ladder, longer-duration elevated water temperatures began to form a “thermal barrier” to upstream migrating salmon and steelhead, slowing and/or stopping adult fish migration upstream.

During 2015 fish perished throughout the West in rivers with and without dams due to elevated water temperatures. A total of 510,705 sockeye that originated in numerous watersheds passed Bonneville Dam on the lower Columbia River. Snake River-born sockeye comprised about 4,069 of the more-than-510,000 sockeye run, or less than one percent of the total run, as confirmed by Passive Integrated Transponder (PIT) tag analysis.

By the time the survivors of those initial 4,069 Snake River sockeye reached Ice Harbor Dam, the first dam encountered on the lower Snake River, 74.1 percent had perished in the lower Columbia River, and their numbers were reduced to 1,052 Snake River sockeye to migrate up the Snake River.

In response, the Corps developed both an interim solution to the thermal barrier in 2014-2015, and installed permanent cooling systems at Lower Granite Dam in 2016 and at Little Goose Dam in 2017.

 Using Dworshak reservoir water to cool the lower Snake River

The Corps keeps the tailwater (water just below the dam) at Lower Granite Lock and Dam at 68 degrees Fahrenheit or cooler to benefit fish passage of Endangered Species Act (ESA) listed salmonids during summer months. The Corps seasonally releases cool water from Dworshak Dam Reservoir on the North Fork of the Clearwater River. Importantly, Dworshak’s cool water “stratifies” deeper in Lower Granite reservoir, the first reservoir it reaches in the lower Snake River. This deeper, cooler water subsequently becomes mixed with warmer water below Lower Granite Dam as it flows downstream.


This process of releasing cool Dworshak water to benefit fish passage in the warmer water of the Snake River is referred to as “flow augmentation.” The Corps implements this program annually, beginning in early summer when water temperatures increase and ending in early fall when water temperatures begin to cool naturally.  

 The Temperature Differential or Thermal Blockage Problem at Lower Granite

The surface water from the forebay (water just upstream of the dam) at Lower Granite provides flow for operation of both the fish ladder and the adult fish trap built into the fish ladder. The fish trap is used to safely and efficiently collect selected adult fish for research or transportation to hatcheries.

In 2013, 2014 and 2015, when the reservoir surface water warmed significantly for a prolonged time period, a warm-water thermal blockage prevented adult fish trap operation and inhibited adult fish from entering the fish ladder.

Many salmon that had migrated upstream through Little Goose Lock and Dam, the next dam downstream, never successfully passed Lower Granite. This raised awareness throughout the regional fisheries management community of the need to find a solution for this issue.

 Adult Fish Ladder Temperature Improvement

The Corps had previously undertaken a study to determine alternatives to resolve elevated fish ladder temperatures at Lower Granite, but hadn’t received funding to pursue a solution. Recent thermal barriers to adult fish migration led to funding for construction of two permanent “intake chimneys” at Lower Granite to cool the adult fish ladder, plus the adult fish trap built into the fish ladder. 

The permanent intake chimneys are large vertical structures bolted to the upstream face of the dam.

There are two chimneys, one on either side of the upstream end of the fish ladder where adult fish normally continue their upstream migration into the forebay in the reservoir. They both draw water from about 66 to 70 feet deep, depending on forebay elevation.

1. Supplemental water intake chimney - One intake chimney covers the pipe entrance that provides supplemental water to the fish ladder at ‘Diffuser 14’ starting about 150 feet downstream from the top of the ladder. It also routes water to the adult fish trap.  

2. Pump intake extension chimney - A second chimney was constructed to extend an existing pump intake. This pump intake provides water to the upstream end of the fish ladder by spraying cooler water in a circular pattern to cool both the immediate forebay area and the fish ladder itself.

The two intake chimneys provide a permanent way to pull cooler water from deep in the reservoir into both the Diffuser 14 pipe and the pump intake. As a result, beginning in 2016, cooler water from approximately 70 feet deep in the dam forebay will now supply both the fish ladder and the adult trap during hot summer months.

Previously, in 2014 and 2015, prior to installation of the two permanent intake chimneys completed in early 2016, the Corps rented pumps temporarily during the summer months to bring cooler water up from deep in the reservoir to benefit operation of both the fish ladder and the adult trap.

The Corps also modified Lower Granite powerhouse and spillway operations to enhance attraction of adult fish to the ladder. Powerhouse operational changes included changing which turbine units were operated. Spillway operational changes included closure of the spillway weir. These manipulations created a cooler water flow profile downstream of Lower Granite to best attract adult salmonids to the fish ladder entrance. This temporary 2014-2015 solution was only partially successful in eliminating the thermal barrier issue. Availability of funding allowed construction of the new permanent solution completed in 2016.

During 2016, monitoring of the completed permanent improvements will document the system’s effectiveness. Monitoring efforts will include use of both tagged fish and temperature sensors. The tagged fish will be monitored to confirm this improvement works as designed to help adult salmon and steelhead continue their migration upstream more expediently through the fish ladder into the reservoir above the dam.

  A pump extension intake chimney provides water to the upstream end of the adult fish ladder. The semi-circular pipe sprays water to cool the fish ladder and adjacent forebay area. U.S. Army Corps of Engineers photo.

  Circular cool water spray at upstream end of Lower Granite Dam adult fish ladder. U.S. Army Corps of Engineers photo.

  A supplemental water intake chimney provides supplemental water to “Diffuser 14” starting about 150 feet down from the top of the fish ladder, plus water to the adult fish trap. U.S. Army Corps of Engineers photo.

Snake River Dams: Project Information

Current Fish Passage at the Lower Snake River Projects

The four lower Snake River dams were designed with features to aid the migration of both juvenile and adult fish. In the last 25 years, the Corps has consistently investigated and adopted new technologies for maximizing the survival of juvenile and adult fish.

Juvenile fish survival past the dams has increased through extensive dam modifications, such as surface passage, juvenile bypass systems, and more effective and efficient spill operations. Through their turbine improvement program, the Corps of Engineers has made improvements to turbine design and modified operations to improve fish survival through the turbines.

For adult fish returning from the Pacific Ocean to spawn, fish ladders and devices to attract fish to the entrances of the ladders are the primary aid to their passing the dams. Fish ladders have been in place since the dams were built in the 1960s and early 1970s. Improvements to these ladders have been made at all four dams.

Overall, these improvements are making a positive impact on salmon and steelhead returns.