Lake

A natural low terrain

gouged by ice

water flowing

Downstream

deep and clean

free of sediment

Fills in and forms

a wetland

eventually meadow

(d'après une étude parue sur le site du
Washington State - Department of Ecology :
« Life History of a Lake »

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Life History of a Lake

A question frequently raised regarding aquatic plants is why some lakes have them in abundance and others do not. An answer to this question lies in the explanation of the lake's aging cycle.

Most of our local lakes came into being as a result of activity of glaciers in the most recent ice age, approximately 10,000 years ago. A lake bed is a natural depression or low spot in the terrain. In the Puget Sound lowlands they were often gouged out by movement of glacial ice. These depressions then became holding basins for water of the drainage area as it flowed toward sea level. As a lake detains water on its way downstream, it also becomes a settling pond for sediment. Part of the sediment that settles in lakes is carried in by the flow of streams or other runoff, and part comes from the accumulation of the remains of organisms in the water and near the shoreline. Aquatic life includes visible plants and animals and also multitudes of microscopic plants and animals that can, over time, add significantly to the accumulation of sediment in the lake by dying and settling to the bottom. The microscopic plants in the water are nourished by plant nutrients (phosphorus and nitrogen) that originate in the watershed and are washed into the lake. Within the lake, a portion of these nutrients can be recycled indefinitely, while more continue to be washed in from the lake basin. Over time sediment accumulates in the lake as productivity gradually increases.

When first formed, many of the lake beds were deep and "clean" (mostly free of sediment). Sediments that were first deposited were silty and had little organic material because there was little life in or around the lakes. Over time, hundreds or thousands of years, the sediment deposits became deeper and more favorable to the growth of rooted, aquatic plants (macrophytes). As these plants (and the microscopic life in the water, now also more abundant) died back at the end of each growing season or life cycle, they enriched the sediments with organic material. Since macrophytes tend to grow better in organically richer sediments, this process set up a cycle of more growth in the lake causing richer sediments and these in turn favored even more growth of macrophytes.

As both microscopic and rooted plants continue to grow in increasing volumes and to enrich the bottom sediments of the lake, the lake ultimately fills in with silt and partially decomposed plant and animal material, forming a wetland and eventually a meadow. This occurrence marks the end of a lake's life.

A lake's life generally extends over several hundreds or thousands of years under natural conditions. It can, however, be significantly shortened by human activities such as improper construction practices that disturb the soil and make it susceptible to erosional processes. As erosion increases in the watershed, quantities of sediment transported to the lake also increase. Consequently, the lake fills in faster and its life expectancy is reduced. The rate at which the process occurs varies with the size, shape, and flushing rate of the lake. Human beings introduce to the watershed nutrient-rich materials such as fertilizers and wastes. Installing hard surfaces (roofs, roads, pavement) makes it easy for these nutrients to be washed into streams and lakes. These activities boost the continuing production of organisms that contribute to the filling of the lake as they die and become part of the organic sediment that favors macrophyte growth.