Lake Washington Ecosystem

With an area of about 21,500 acres, Lake Washington is the second largest lake in our state (after Lake Chelan). It is 22 miles long and averages about 2.5 miles wide. The surface is about 20 feet above sea level, and the lake has a maximum depth of 214 feet with a mean depth of 108 feet. Except for Andrews Bay and Union Bay, the lake has generally steep sides and the floor is about 200 feet deep. The lake is currently classified as a mesotrophic lake, with relatively clear waters and intermediate levels of nutrients that support aquatic plants and fish.

The lake ecosystem is the oldest ecosystem in the park, evolving shortly after glacial recession. For over 13,000 years Lake Washington drained via the Black River in Renton. In 1916, the Montlake Cut of the Lake Washington Ship Canal was opened, lowering the mean lake level by 8.8 feet and re-routing its drainage into the ship canal, so that the Black River now exists only as a small creek. Lowering the lake added new shorelands to Seward Park, and all along Lake Washington Boulevard, but it also eliminated many plant communities and fish populations.

The Lakeshore

After the present shoreline was created in 1916, both natural re-vegetation and human intervention worked to make the current shoreline plant communities. In the 1930s, the shoreline was graded to make the loop road. Ornamental trees such as Lombardy poplar, cherries, catalpa, and others were planted in many places along the shore, but most of the shoreline appears to have revegetated naturally. Both the forest and the marshy areas drained by the lake lowering probably served as sources of seeds. Douglas firs and thimbleberries came from the forest, and Oregon ashes, Sitka willows, rushes, sedges, cattails, and others probably came from nearby wetlands. The new shores also provided opportunity for invasive species such as Himalayan blackberry, reed canary grass, and yellow loosestrife.

The Plankton Cycle

Phytoplankton populations in the lake, including diatoms, dinoflagellates, and green algae, as well as cyanobacteria, have a complex cycle dependent on light, nutrients, and zooplankton populations. Phytoplankton increases from January to March with lengthening days through the spring, resulting in a “spring bloom” that peaks from April through June. Diatoms are the most abundant group of phytoplankton during the spring bloom. The phytoplankton population decreases in summer as phosphorus becomes limiting for growth and the zooplankton population increases. Zooplankton are microscopic crustaceans and other microscopic organisms that feed on phytoplankton. The composition of the phytoplankton shifts to increasing amounts of dinoflagellates, green algae and cyanobacteria during the summer, possibly because the lake becomes limited for silica, which is needed by diatoms to make their glasslike coverings. From July through October levels of phytoplankton,
phosphorus and zooplankton are more constant, perhaps because much of the phosophorus available comes from zooplankton excretions. In November and December both phytoplankton and zooplankton decline as light becomes


Several groups of invertebrates are found in the benthos. Hydras, flatworms, nematodes, tardigrades (waterbears), rotifers, water mites, four species of oligochaete annelid worms, and midge larvae (Chironomidae), of which there are 13 species, make up the majority of the biomass in the benthos. In addition to midge larvae, other insects found in the lake include dragonfly nymphs, mayfly nymphs, stonefly nymphs, and caddisfly larvae. Caddisfly larvae are noted for the distinctive cases many of them build out of sand grains, bits of leaves, twigs, or other materials for protection. In all of these insect groups, only the juvenile stages are aquatic, and the adults emerge into the air. Caddisfly adults typically feed on liquids and remain in the area. Adult mayflies and stoneflies are usually short-lived and often do not feed. In contrast, the predatory dragonflies often survive the entire summer and into the fall.

Crustaceans in Lake Washington include the signal crayfish (Pacifastacus leniusculus), amphipods, ostracods, copepods, cladocerans, and the opossum shrimp Neomysis mercedis. Signal crayfish populations are healthy and support seasonal harvesting by human sport fishers. They typically are 3 inches long but can reach 7 inches.

Snails include Gyraulis (ram’s horn snails) and Lymnaea (pond snails), along with Viviparus georgianus (sometimes called the banded mystery snail), a recent invader from the American southeast that is commonly found along the shores of Andrews Bay. This snail prefers a silt-mud substrate and may be favored by the growth of Eurasian milfoil and its accompanying silty substrate. Viviparus matures in one to three years.

The tiny freshwater clam Pisidium casertanum, known as the pea clam, pea cockle or pea mussel, reaches only ~5 millimeters in length and often attaches to aquatic vegetation. It is one of the most widespread freshwater clams in the world. In Lake Washington these clams have a one-year life-span and reproductive cycle. Also found in the lake is the invasive Asian clam Corbicula fluminea. In southern China it is widely eaten, and is known as the golden clam or the prosperity clam. It has been suggested that the golden clam was introduced to American lakes and rivers as bait. The golden clam grows up to ~2 inches (5 cm), but is usually less than 1 inch. It can self-fertilize, allowing one or a few individuals to establish a population.

The freshwater mussel Anodonta kennerlyi, known as the western floater, can grow 7 inches long, inhabits the lake bed, and was a food source for the Lake People. Freshwater mussels are not closely related to freshwater clams, saltwater mussels, or the invasive European zebra mussels, and have a unique life cycle. They have separate sexes, and males release sperm into the water during breeding. Females must filter out the sperm for fertilization in their marsupium, a special part of their gill. Fertilized embryos develop into larvae called glochidia, which resemble microscopic mussels and attach to the gills of fish. Salmon, trout, prickly sculpins, sticklebacks and other fish can serve as hosts for the glochidia and transport these hitchhikers to new locations, sometimes leaping waterfalls. Thus though adult mussels may move no more than a few yards during their lifespans of more than 100 years, glochidia can spread wherever their host fish can go. Eventually they drop off and settle in the substrate, which, whether in a lake or river, should be in a low-gradient substrate stable toward current and erosion. If in a suitable location, a glochidium will grow rapidly to increase its chance of survival. Because sexes are separate, even if a glochidium becomes an adult it cannot reproduce unless a mussel of the opposite sex is nearby. Though once used for food by native Americans, mussels are in serious decline, concentrate pollutants in modern rivers, and reportedly are also now unpalatable.


Lake Washington is home to about 40 kinds of fish, about a quarter of which were introduced. Changes in fish from lowering the lake and re-routing its outlet in 1916 were most likely dramatic, but are poorly documented. Eutrophication of
the lake in the 1950s and 1960s brought further changes, as did lake clean-up through better sewage treatment in the later 1960s and 1970s. A survey in 1975-1976 near Renton found half of the fish were longfin smelts, while three-spine sticklebacks and yellow perch, an introduced species, together made up another third. Smelts still made up half the fish in surveys from 1989-1992 while sticklebacks declined to less than 5%. In a 2005 survey, however, three-spine sticklebacks
made up three quarters of the fish in the lake, indicating continuing dynamics of fish populations.

In April Seward Park visitors may encounter writhing masses of peamouth chub spawning by the thousands for one or two days in Andrews Bay and elsewhere along the Seward Park shoreline. Kelsey Creek is also a major spawning site for peamouths, which enter the creek briefly to spawn on one to four occasions between April and mid-June. After spawning, the peamouths leave the creek and return to
Lake Washington. Bellevue’s Stream Team improved the habitat for peamouths near the Wilburton Trestle, and has a Peamouth Patrol that helps monitor spawning events. Check out videos here and here!

Salmon are the best known examples of anadromous fish that live most of their lives at sea and return to the Lake Washington watershed to spawn. Seven species of Pacific salmon and trout in the genus Oncorhynchus have been reported in Lake Washington: pink, chum, coho, sockeye, and chinook salmon, as well as steelhead
and coastal cutthroat trout. Rainbow trout is the same species as steelhead, but rainbow trout stay in freshwater. In addition, four species of trout, or more properly char, in the genus Salvelinus also reside in the lake: the native bull trout and Dolly Varden trout, and the introduced brook trout and lake trout. Pink and chum salmon are uncommon, and populations of bull trout, chinook salmon and the kokanee, a lake-bound version of sockeye, are all considered to be threatened. Kokanee was once the most abundant game fish in Lake Washington, but populations have plummeted in recent years, and kokanee now are found only in Lake Sammamish in very limited numbers. The reasons for their decline are not entirely clear, but undoubtedly include habitat degradation due to urbanization and climate change, which affects lake stratification and the food web.

Some 70% of the Lake Washington shoreline is occupied by bulkheads, seawalls, and other hardscape that is poor habitat for salmon. Since chinook salmon were listed as threatened in 1999, Seward Park has been a major focus of habitat restoration for salmon because it is one of the largest stretches of natural shoreline remaining in the lake. There are three populations of chinook in the Lake Washington watershed: the Cedar River, North Lake Washington, and Issaquah populations. The Cedar River population has the highest quality habitat, but is also considered to be at greatest risk. Juvenile salmon from this population travel past Seward Park on their way to the sea. Four areas in Seward Park have undergone habitat enhancement to provide shallow water, appropriately-sized gravel, and overhanging vegetation that provides shade and insects for food, in order to promote the survival of young salmon. Additional habitat enhancements have occurred at Martha Washington Park and at Chinook Beach, and in the northern part of the lake where they may help the Northern Lake Washington and Issaquah populations.

An important food source for salmon that is also threatened with extinction is the Pacific lamprey. Like salmon, Pacific lampreys are anadromous, with juveniles living as filter feeders in silt beds of rivers for several years before they depart to the sea. As adults they attach themselves by their sucker-like mouths to other fish and live as ectoparasites. Larger fish usually survive being host to a lamprey, and the lamprey eventually returns to freshwater to spawn. Both males and females carry rocks in their strange mouths to build a nest for their young before they spawn and die. Lampreys belong to the primitive jawless fish and are poor swimmers with fatty flesh that makes them good prey for salmon and for many Native American tribes, who have eaten them for millennia. In addition to the Pacific lamprey, the river lamprey and very similar western brook lamprey also are found in Lake Washington and are threatened with extinction. The river lamprey spends only ten or so days as a sea-going parasite before it returns to the rivers to spawn. The western brook lamprey remains in fresh water and does not feed as an adult.

Other vertebrates

Red-eared sliders sunning themselves on a log. Sliders are non-native turtles from the pet trade.

Besides fish, a number of terrestrial vertebrates spend part or much of their time in or on the lake. Turtles, including native western painted turtles but especially the introduced red-eared sliders from the southeastern United States, are abundant in Andrews Bay, where they are fond of basking on logs or the docks. Red-eared sliders, which were formerly sold as pets in dime stores, have become established in lakes and ponds around the world, frequently to the detriment of native turtles. Elsewhere on Lake Washington in Union Bay, spiny softshell turtles, another import from the southeast, are reproducing. Beavers have a lodge by the Lakewood Moorage and frequently visit Andrews Bay to munch on water lilies. Muskrats and river otters are occasionally observed along the park shorelines. In spring and fall spotted sandpipers and other shorebirds stop off in Seward Park during migration, while in winter a variety of ducks, geese, gulls, and grebes, as well as coots, common loons, and double-crested cormorants visit the shores. Mallards and gulls are year-round residents of the park, and herons, kingfishers, ospreys and bald eagles fish the waters through much of the year.