This week, a news story aired about the nearest large city exploring additional sources of drinking water. Sioux Falls, South Dakota, in the middle of the United States, has experienced the second year of drought, and aquifers need to be recharged. Water is a precious resource for humans, plants, and animals.
Also this week, a quiet, recurring water miracle happened on the nearby Lewis and Clark Lake, a reservoir above the Gavins Point Dam on the Missouri River. One of the first visible signs is that some stones along the lake’s edge are coated with ice from the cool air.
Over a mile wide, the lake has a calm surface and light and dark lines on the water. The water, top to bottom in the lake, reaches 39.2 degrees F. The cool water settles to the bottom, causing a fall turnover of nutrients. The movement adds oxygen deep in the lake, where fish will overwinter.
All the shallow water is cooled along the lake’s edges to 39.2 degrees F. The air above the lake stays below freezing for a few days. A skin of ice begins to form on the surface of the water.
Days later, more of the deep lake reaches 39.2 degrees F., the surface to bottom. A layer of ice forms and thickens because the air above the lake stays colder than 32 degrees. As the liquid water becomes a solid layer, it floats because ice is lighter than water. The layer thickens about 1/3 inch a day, dependent on consistent freezing air temperatures above, slow water current, and other factors. Soon much of the lake is covered with a layer of ice, forcing migrating geese and other waterfowl into a small open water area.
Now at the end of the week, the water is quiet, lacking waves and migrating birds. The morning sun glances off the ice in the frigid air across the lake as layers of ice continue to form. Snow and cold air help insulate the ice over the winter. Our precious water resources completes all this in a week.
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