The secret life of lakes - King County, Washington
Ever wonder why you sometimes feel a layer of cold water when you are swimming in a lake? Or why some lakes shrink in the summer while others stay full year-round? The answers are in the thermal properties of water, the hydrologic cycle, climate and their effects on lakes.
What is the hydrologic cycle?
Water moves from the atmosphere to the ground and back again, in a circular pattern called the hydrologic cycle. Clouds hold water vapor, which falls to the earth as rain, snow or sleet. About three-fourths of that water returns directly to the air- either by evaporating from the earth's surface or transpiring from the pores of plants. Much of the water that remains on the earth's surface is stored in streams, lakes, ponds, wetlands, groundwater and oceans. Lakes serve as temporary holding areas for water before it circulates again through the hydrologic cycle.
The water budget
The water in a lake is constantly changing. New water enters from several sources: direct precipitation, surface runoff, flows from rivers and streams, and subsurface flows of groundwater. Water also leaves lakes through several different routes, including surface outflows, direct evaporation and seepage into groundwater. The sum of all the water moving into a lake and the water already present in the lake, minus the water leaving the lake on an annual basis makes up the water budget. Timing, duration and intensity of precipitation have dramatic affects on a lake. At the end of summer, lake waters are generally at their lowest levels. In the fall, as the rainy season begins, soils become saturated, streams begin to fill, and lake levels rise. King County lakes acquire most of their water during the six-month period of cool, wet weather, from October to March.
The time factor
Some lakes stay relatively clean and clear year-round, while others seem to be more vulnerable to pollution, poor water quality, and algae blooms or other signs of increased productivity. Why? It may be because of each lake's hydraulic residence time- the total time that it takes to completely replace the volume of water through natural inflow to, and outflow from, the lake. Lakes with relatively short hydraulic residence times (days or weeks) stay relatively free of algae, because most of these single-celled plants are flushed out before they can grow and multiply. Lakes with long hydraulic residence times (months or years) favor the growth of algae and are more susceptible to the effects of pollution and decaying aquatic plants.
Layers of water
The density of water varies with its temperature. At 39 degrees Fahrenheit, water molecules draw closely together and water becomes as dense and heavy as it can get. At temperatures both higher and lower than 39 degrees, the molecules spread out and the water becomes lighter and less dense. Because ice is less dense than water, it forms at a lake's surface and not at the bottom. If not for the unique properties of water, lakes would be very different systems. Many King County lakes have fairly consistent water temperatures from month to month. But in many of the County's large lakes, water collects in layers of different temperatures and densities- a phenomenon called thermal stratification. During summer months, the water in deep lakes stratifies into three layers:
- The epilimnion is the uppermost layer of water, warmed by the sun. The hypolimnion is the lower, cooler layer near the bottom.
- These two layers are separated by a zone of rapidly changing temperatures and densities called the metalimnion.
There is little mixing across these layers during stratification and each layer retains its unique composition. In the fall, winds blow and waves begin to disturb these layers. The water cools and, in what is called the fall overturn, the temperature zones break down, and the lake's water mixes completely. During the winter, the waters stay mixed, but, come spring, the sun warms the top layer and the lake's waters stratify again.
Watersheds and their lakes
A watershed is the area of land that drains to a lake or other water body. Watersheds vary in size from just a couple of acres to thousands of square miles. The characteristics of a watershed and the land use activities within it have profound effects on lakes. If a hillside is clear cut or a wetland drained, a lake may receive significant pollution loads or its water cycle may be altered. Changing from undeveloped to residential land uses may result in a 700-percent increase in phosphorus loading to a lake. Much of this increase can be traced to the loss of forests and the removal of plant material- both of which naturally bind phosphorus, keeping it out of lakes and other water bodies. Other sources of phosphorus in lakes include failing septic systems and activities such as car-washing and landscaping. Watersheds and human activities determine how and at what rate water enters the lake:
- Larger watersheds accumulate more water that may flush through lakes at a greater rate. In watersheds with wetlands, lake levels will fluctuate more slowly, because wetlands act as sponges, soaking up and gradually releasing precipitation.
- As watersheds become urbanized and acquire more rooftops, parking lots and other places where water can't sink in or infiltrate, lakes receive more surface runoff all at once-and more pollutants carried by the runoff.