Dredging the Lake

Young People Swimming in Pine Lake, ca 1940s

A young lake is said to be oligotropic. It is usually relatively clear, high in dissolved oxygen, and deep and receives few nutrients, thereby supporting little plant and animal life. As nutrients increase, together with siltation due to the acts of humans and nature, plant and animal life increases. The lake then begins to mature and is referred to as a balanced mesotrophic lake. The continued siltation and accumulation of organic matter begin to fill up the lake, making it shallower. This, together with proper nutrients, increases the growth of aquatic plants, particularly algae, and the lake becomes mature, or eutrophic, with low water transparency, large organic deposits colored brown or black, and often hydrogen sulfide odors. If there is an excess of nutrients, the algal growths greatly increase ("bloom"), die, and decay. The decay process uses up more oxygen to the point of there not being enough for other forms of aquatic life. As the growth and decay progress, the lake fills with organic matter and and silt to become a marsh and, eventually dry land.

     -- Salvato, Nemerow, & Agardy, Environmental Engineering, John Wiley & Sons, 2003, p. 551

Dredging the Lake

The average lifespan of a man-made lake is about 75 years. During that time sediment carried in by the lake's headwaters slowly settles on the lake bottom, making it more and more shallow. As the lake's depth increases the temperature decreases; this leads to increased algal and bacterial growth which can pose a hazard to swimmers and fishermen. Eventually the lake will fill with no longer be a lake.

Our lake was created in the mid-1930s-- 75 or so years ago. The eastern portion was purposefully shallow, the western portion less so. Snapfinger Creek apparently fed directly into the lake-- along with all its silt. At some point-- I believe it was the early 1940's-- the city created a berm along the south side of the lake, creating a separate channel for Snapfinger and a flume for inflow into the lake.This certainly reduced sedimentation, but by the beginning of the twenty-first century the lake was moving inexorably toward eutrophy. The lake had become warmer as it had grown more shallow, and bacterial counts often precluded swimming.

The solution to this problem is, clearly, dredging-- draining the water from the lake, removing accumulated silt and hopefully making the lake a bit deeper than it was originally, rebuilding the berm along the south side of the lake, refilling the lake, and restocking it with native fish.

Happily, this is about to happen.

In future posts I'll write more about this project and document its progress with photos.