Geologic and Human Events

MOST LAKES were created by geologic events. The vast lake-dotted and marshy landscapes found in North America were formed by glacier action 10,000 to 20,000 years ago. Glaciers formed lake basins by gouging holes in loose soil or bedrock, by depositing material across streams beds, or by leaving buried chunks of ice whose melting shaped lake basins. More recently, humans and other animals have created lakes and reservoirs by damming rivers and streams.

Lakes constantly undergo evolutionary change, reflecting the changes that occur in their watersheds. Most are destined to fill in with remains of lake organisms and with 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.

Hydrologic Cycle: How the water comes and goes

In Washington, about three-fourths of the precipitation that falls reenters 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 is the principal cause of 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 modify some fluctuations, but varying lake levels are normal.

Stratification: Layers of a lake

Water in lakes in temperate climates 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. Above and below that temperature, water expands and becomes less dense. Many lakes stratify in winter because ice covers the lake surface. Lakes in areas with milder winters do not stratify during the winter. In spring, as ice melts, the surface waters warm; sink; and mix with the deeper water, a process called spring turnover.

As summer progresses, the temperature difference (and density difference) between surface and bottom water becomes more distinct, and most lakes form three layers. The upper layer, the epilimnion, is characterized by warmer (less dense) water and is the zone of light penetration, where the bulk of productivity or biological growth occurs.

The next layer, the metalimnion or thermocline, is a narrow band--colder than the upper and warmer than the lower waters--which helps to prevent mixing between the upper and lower layers.

The (third) bottom layer, the hypolimnion, has much colder water. Plant material either decays or sinks to the bottom and accumulates in this stagnant layer.

During the fall turnover, surface waters cool until they are as dense as the bottom waters and wind action mixes the lake so that water temperature from surface to bottom is the same.

Oxygen: Essential to lake life

The presence of oxygen in lake water determines where organisms such as fish and zooplankton are found. 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 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.

Nutrients: Phosphorus and nitrogen

Plants require phosphorus and nitrogen for growth. The concentration 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 is added to the lake from sewage treatment plants, urban or farmland runoff, lawn or garden fertilizers, septic tanks or other watershed or outside resources, or even if it is released from phosphorus-rich lake bottom sediments, more algae will grow.

Under certain conditions, especially when oxygen is absent from bottom waters, phosphorus is released from bottom sediments into the overlying water.

In turn, algae clouds water clarity and decreases the depth of light penetration. By measuring the phosphorus concentration, algae abundance (by chlorophyll analysis), and water clarity (by Secchi disk measurement), the so-called trophic status is identified.

A eutrophic, or nutrient-rich lake tends to be cloudy or green with algae and may have limited oxygen in the hypolimnion. An oligotrophic lake is relatively nutrient-poor, is clear, and has adequate dissolved oxygen in the hypolimnion. A mesotrophic lake is between the two. Factors vary from lake to lake, and designations of lakes as eutrophic; mesotrophic; or oligotrophic tend to be subjective.