The following information is also available in a PDF format from the Washington Lake Book. For information about how to monitor lakes and streams, see also the Citizen's Guide to Understanding and Monitoring Lakes and Streams. Water on the Web (http://waterontheweb.org/under/lakeecology/index.html) has an excellent site that explains the ecology of lakes with photographs that explain lake concepts.
There are nearly 8,000 lakes in Washington. Lakes originate in a number of ways. Most of Washington's lakes resulted from glacial and river or stream action. In the Puget Sound lowlands, many lakes occupy depressions in the surface formed by glaciers during the ice age 10,000 to 20,000 years ago. Glaciers formed lake basins by gouging holes in loose soil or bedrock, depositing material across streams beds, or leaving buried chunks of ice whose melting shaped lake basins. Lakes in the higher mountains occur in basins cut by local alpine glaciers.
Rivers also form lakes. Oxbow lakes formed as a cutoff of a river meander. In the Columbia basin, lakes in the coulees of channeled scablands were formed by gigantic, catastrophic floods. A few lakes, like Battle Ground Lake in Clark County, occur in the craters of extinct volcanoes. In the lower Columbia basin, irrigation seepage and runoff has created numerous ponds and lakes in depressions. Quarries and other excavations that people have abandoned may fill with water and become lakes. Kress Lake in Cowlitz County is an example of this type of lake. Humans have created reservoirs (artificial lakes) by damming rivers and streams. Beavers also play a role in lake formation by building dams. (Information from the Primer on Lakes in Washington, Water Supply Bulletin 49. Prepared by the United State Geological Survey, 1978)
Lakes constantly undergo evolutionary change, reflecting the changes that occur in their watersheds. Most lakes will eventually fill in with remains of lake organisms and silt and soil washed in by floods and streams. These gradual changes in the physical and chemical components of a lake affect the development and succession of plant and animal communities. This natural process takes thousands of years. Human activities, however, can dramatically change lakes, for better or worse, in just a few years.
In Washington, about three-fourths of the precipitation that falls re-enters the atmosphere by transpiration from plants and evaporation from the earth's surface. Much of the remaining water seeps or soaks into the ground water and moves underground toward lakes and rivers. Water that runs off the land surface also enters rivers and lakes.
Lake levels vary from season to season and year to year. Precipitation causes most lake level fluctuation. If rainfall decreases, the lake level falls; if rainfall increases, the lake level rises. However, the lag between precipitation and lake level change varies from days to years, depending on the lake. Dams can equalize some of the fluctuations, but lake levels often vary under normal conditions.
In temperate climates, lake water tends to stratify or form layers, especially during summer, because the density (weight) of water changes as its temperature changes. Water is most dense at 39 degrees Fahrenheit (4 degrees centigrade). Above and below that temperature, water expands and becomes less dense. Many lakes stratify in winter because ice covers the lake surface. In spring, as ice melts, the surface waters sink and mix with the deeper water, a process called spring turnover. Lakes in areas with milder winters do not stratify during the winter.
As summer progresses, the temperature difference (and density difference) between surface and bottom water becomes more distinct, and most lakes form three layers.
During the fall turnover, surface waters cool until they become as dense as the bottom waters and wind action mixes the lake so that water temperature from surface to bottom is the same.
The presence of oxygen in lake water determines where organisms such as fish and zooplankton live. In spring, when the lake water is well mixed, oxygen is usually present at all depths and organisms may be distributed throughout the lake. In the summer, under stratified conditions, little or no oxygen is produced in the hypolimnion. Available oxygen is consumed through decomposition of plant and animal material, and oxygen levels may become too low for fish who must move to the top layer, or epilimnion. If these conditions are prolonged and the upper waters become too warm, cold-water fish such as trout may become stressed and eventually die.
In the fall, the lake layers break down and turnover replenishes oxygen to the bottom waters. The formation of ice in water reduces the supply of oxygen to the lake from the overlying air. If oxygen levels fall too low, fish and other aquatic life may die
Plants and algae require phosphorus and nitrogen for growth. The concentrations of these substances in water and sediments control the total amount of plant matter that can grow. In most lakes, phosphorus is the least available nutrient; so its abundance, or scarcity, controls the extent of algae growth. If more phosphorus enters the lake more algae will grow. Nutrients can come from lawn or garden fertilizers, sewage treatment plants, urban or farmland runoff, septic tanks or other outside sources. Under certain conditions, especially when oxygen is absent from bottom waters, phosphorus is released from bottom sediments into the overlying water.
Nutrients fuel algae growth. Elevated populations of algae cloud water clarity and decrease the depth of light penetration. By measuring the phosphorus concentration, algae abundance (by chlorophyll analysis), and water clarity (by Secchi disk measurement), a lakes nutrient-status (the trophic status) is identified.
Factors vary from lake to lake, and designations of lakes as eutrophic; mesotrophic; or oligotrophic tend to be subjective.
Over its lifetime, a lake progresses from a more nutrient-poor (oligotrophic) to a more nutrient-enriched (eutrophic) state. When nutrients such as phosphorus and nitrogen wash into a lake, they fertilize the lake. This encourages algae and larger plants to grow. As plants, and the animals that feed on them, die and decompose, they accumulate on the lake bottom as organic sediments. After hundreds or thousands of years of plant growth and decomposition, the lake may more closely resemble a marsh or a bog. This natural transition process is called eutrophication.
Lakes also receive nutrients from human activities. This can literally make a lake old before its time. This accelerated transition is called cultural eutrophication. Nutrients from fertilized yards and gardens, agricultural areas, stormwater runoff, urban development, failing septic systems, land clearing, municipal and industrial wastewater, runoff from construction projects, and recreational activities contribute to accelerated enrichment or eutrophication.
Sedimentation is closely associated with eutrophication. Wind and water move soils from the watershed into a lake. The soils settle on the bottom of a lake, and the lake becomes increasingly shallow as part of the natural filling of the lake.
Sedimentation is greatly accelerated, however, by human activities that leave the soil exposed without vegetation for extended periods. Land development, construction and agricultural activities, or farming steep slopes leaves soils vulnerable to erosion. Sedimentation is best controlled through soil and water conservation practices, maintaining vegetation on soils, and use of best management practices during construction.
Copyright © Washington State Department of Ecology. See http://www.ecy.wa.gov/copyright.html.