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4.8: The Lifetime of Lakes

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    14333
  • How long do lakes last? There’s an inevitable tendency for lakes to disappear, by sedimentation to the point where the lake basin is occupied by sediment rather than water, so the water surface is replaced by land surface, perhaps with a stream or river flowing across that land surface.

    One cause for the disappearance of lakes is the mechanical deposition of inorganic mineral sediment (gravel, sand, and mud) carried in mainly by streams and rivers feeding the lake, or to a much lesser extent by wind or by mass movements of soil down slopes to the edge of the lake. The sediment load of a stream feeding the lake tends to become segregated into coarse material deposited near the mouth of the stream to form a delta, and fine material deposited from suspension widely over the lake bottom (Figure 4-13).

    Fig. 4-13.png
    Figure 4-13. Sediment deposition in lakes.

    The other important cause for the disappearance of lakes is biogenic sedimentation of non-decomposed organic matter, mainly plant material, on the lake bottom. This process is called eutrophication. The important factor here is availability of oxygen in the bottom waters.

    • If the biogenic productivity of the lake (mainly at the surface) is relatively low and the vertical mixing of water in the lake is relatively high, then the bottom water is well oxygenated, and most or all of the organic matter that settles to the bottom is decomposed before it has a chance to build up an organic bottom- sediment deposit.

    • On the other hand, if the biogenic productivity is relatively high and the vertical mixing of water in the lake is relatively low, then the bottom water is deficient in dissolvedoxygen, and a deposit of organic sediment builds up on the lake bottom.

    Because the vertical circulation is controlled almost entirely by physical effects, as discussed in earlier sections, the critical factor in eutrophication is biogenic productivity, which in turn is a function of supply of dissolved nutrients in the water entering the lake. In most lakes, phosphorus is the limiting nutrient. One of man’s strongest effects on lakes (in urban areas, suburban areas, and rural agricultural areas) is the introduction of much greater concentrations of nutrients, especially phosphorus, leading to greatly accelerated eutrophication.