Salmon live in a topsy-turvy world upstream of the Ballard Locks

Through much of the year, migrating salmon headed to or from Lake Washington encounter a man-made estuary like nothing that exists in the natural world. This estuary — a place where freshwater mixes with seawater — is a concrete dam with special openings for ships to pass through. Freshwater lies on one side of the formidable structure, saltwater on the other.

Not far from Puget Sound in Seattle, the Hiram M. Chittenden Locks, better known as the Ballard Locks, includes a spillway, fish ladder and complex plumbing system that complicates salmon migration. 

Adult salmon make a rapid transition from saltwater to freshwater at the locks, either by swimming up the fish ladder or by passing through with the ships. Their offspring make an equally rapid transition to saltwater on the return trip, with large numbers passing over the spillway through a “smolt flume” installed each spring. Migrating juvenile salmon, called smolts, are attracted to the high-velocity flows through the flume.

When the Ballard Locks were completed in 1916, the world was turned upside down for the salmon, which previously migrated south out of Lake Washington into the Duwamish River. Their new access to and from Puget Sound took them through Lake Union and a man-made canal far to the north of their previous route.

“Salmon are incredibly adaptive,” said Jason Mulvihill-Kuntz, salmon recovery manager for the Lake Washington/Cedar/Sammamish Watershed. “Through the generations, they have learned a new pathway to get back. They have developed a life strategy for dealing with their new conditions.”

Altered ecosystem

For smooth operations through the Ballard Locks, engineers decided to lower Lake Washington by nine feet to match the water elevation of Lake Union. In doing so, the lake’s surface area declined by 7 percent, and the total shoreline was reduced by more than 10 miles. [See related story: Will Ballard Locks withstand a major earthquake?]

Freshwater marshes were diminished from an estimated 1,136 acres to just 74 acres. Water levels in the connected Lake Sammamish system also dropped, destroying a wetland complex along the slow-moving Sammamish River.

On the other hand, new freshwater wetlands were created in Union Bay, Portage Bay, Juanita Bay and Mercer Slough, though much of that area has been taken over by development.

Today, the Lake Washington watershed is home to Chinook, coho and sockeye salmon, along with steelhead, rainbow, cutthroat and bull trout. A variety of other fish also can be found in the lake.

Chinook salmon, listed as threatened under the Endangered Species Act, consists of two stocks. One is associated with the Cedar River, which enters the south end of Lake Washington. The other is associated with the Sammamish River, which enters the north end of the lake. Upstream of the Sammamish River lies Lake Sammamish with a fish hatchery farther upstream on Issaquah Creek. Since 1937, the hatchery has helped maintain the Sammamish Chinook population.

Like Chinook, coho are sorted into two stocks, though coho prefer smaller streams. One group of coho is associated with Cedar River and its tributaries, while the other is associated with the Sammamish River plus all the small streams that drain into Lake Washington. Hatchery plantings ended in 1970, except for small educational programs.

One steelhead stock is listed for the entire Lake Washington watershed. Steelhead, now listed as threatened, were planted in streams from 1933 through 2001. Hatchery plants ended in the Cedar River system in 1993.

Sockeye salmon, which depend on a lake for their growth, probably existed in Lake Washington before construction of the Ballard Locks, according to biologists, but their numbers were probably low until the 1930s. That’s when area residents began to plant sockeye fry from Baker Lake into numerous tributaries draining into Lake Washington. The hope of creating a sustainable sockeye run was soon fulfilled.

By the 1960s, the run had grown into one of the largest sockeye runs in the lower 48 states, big enough to allow a sockeye fishery. Ironically for the environmental movement, many salmon biologists have concluded that Seattle’s discharge of sewage into Lake Washington may have helped the sockeye. They say nutrients from the wastewater triggered the growth of plankton, which became an almost unlimited supply of food for the fish. Meanwhile, a dense growth of green algae enveloped the lake, helping them hide from predators.

Cleaning up Lake Washington may have reduced the survival of young sockeye, which generally spend up to 18 months in the lake. But other factors were taking a toll as well — not just for sockeye but for Chinook, coho and steelhead.

Urbanization of the Lake Washington watershed, including deforestation,increased winter flows in nearly every stream, destroying eggs buried in the gravel. Roads altered the streams, wiped out wetland habitat and dumped toxic pollution into the water. Loss of tree cover warmed the streams. All these factors change from year to year, but together they have taken a severe toll on salmon survival in the streams, biologists say.

Challenges for young salmon

In Lake Washington itself, a growing number of predatory fish has been identified as a major problem — especially for young sockeye, which spend more time in the lake than the other species. Studies have shown that an average of 97 percent of the young sockeye that go into the lake get eaten before they can migrate to sea.

A new study started last year has confirmed that cutthroat trout and northern pikeminnow are major predators in Lake Washington. Smallmouth bass, previously thought to be less of a concern, are now being reconsidered as a major predator, because their numbers in the lake and ship canal are large and growing larger. And walleye, a relatively new arrival to the lake, could become a truly worrisome predator, because they are able to hunt in low light, unlike most other predatory fish in Lake Washington.

Researchers are seeking funding from the Legislature to continue the predator study into next year, which could lead to efforts to protect the young salmon. Ideas could include reducing the predator populations or physically moving the young fish to safer areas.

Another challenge for young fish is the highly developed shoreline of Lake Washington. In a natural system, juvenile salmon migrate along the shoreline, pausing to feed in protected areas. The construction of bulkheads, docks and other structures has eliminated much of the shoreline habitat. Now, restoration projects are being planned to provide fish with at least limited refuge.

Young salmon and steelhead that can survive the challenges in the streams and the lake must find their way into the Lake Washington Ship Canal. For salmon, most of the outmigration occurs between April and July.

The small salmon still face additional predators in the seven-mile journey through Lake Union and the ship canal. Though they would prefer to stay near the surface, warm temperatures often cause them to dive deep to find cooler water.

As the fish approach the locks, they are confronted with a mix of salinity, temperature and oxygen changes. Saltwater that washes through the dam when the locks are opened forms a wedge of heavy, high-salinity water on the bottom of the canal upstream of the locks. This wedge varies in length and thickness, depending on flows through the locks. Because the water is cooler, small fish may hang out there for a long time before attempting to pass through the locks.

To reduce harmful effects of saltwater on the freshwater environment, a barrier can be raised from the bottom of the larger locks to reduce flows of saltwater — except when deep-draft ships pass through. Heavier saltwater also is captured in a drain just upstream of the locks and piped to the fish ladder, where it increases the flow to attract more adult salmon and steelhead.

Salmon confront the locks

At the locks, juvenile salmon are attracted by the high flows of water through the smolt flumes, which are installed during the migration period. Some of the little fish inevitably follow ships through the locks, while others take a more hazardous route. If the surface water is warm, for example, the fish may go deep and get sucked into the intake culvert used to fill the locks. Continuing studies are aimed at figuring out which routes are taken under various conditions.

Like the saltwater “wedge” upstream of the locks, a freshwater “lens,” some 3 to 10 feet thick, floats on the saltwater downstream from the locks, its length dependant on flows and tidal conditions. These mixed waters, upstream and downstream of the locks, are the only “estuary” the salmon have available, as juveniles make a dramatic physiological transition from freshwater to saltwater, and adults reverse the process.

Both juvenile and adult salmon — which migrate at different times of the year — show no urgency to leave the locks as they make this transition. They may hang out upstream or downstream of the structure or even pass through the locks more than once. Smolts, for example, may go downstream over the spillway then return through the locks alongside ships headed for Lake Union. Adults may go up the fish ladder and then return with ships headed for Puget Sound before they decide to keep on going.

High temperatures through the ship canal have been shown to slow the migration of adult salmon, which seem to be waiting for cooler water that does not come. Delays can affect their ability to spawn successfully. Warm water can reduce their oxygen supply, sap their strength and make them more susceptible to disease. Engineers have been studying ways to cool off the ship canal, possibly by piping cold, deep lake water out of Lake Washington.

An ongoing experiment involves operating the locks even when no ships are going through. These so-called “false lockages” have increased the oxygen supply and lowered the temperature upstream where they fish hang out, but they have also introduced more seawater into the ship canal. The jury is still out on the tradeoffs associated with the process, said Scott Pozarycki, a fish biologist for the Army Corps of Engineers.

“The future of false lockages and the manipulation of temperature though operational changes is something we continue to investigate,” Pozarycki said. “Temperature is a major challenge throughout the Lake Washington basin.”

Chinook, like all species of salmon, face a multitude of challenges, he noted.

“This (Chinook) stock has persisted through 100 years of evolution,” he said, “but we’re still not sure if it’s a very robust stock, so we’ll continue to look for answers.”