Why are stagnant water bodies often shallow, swampy and overgrown?

1) in a stagnant reservoir, dead organic matter settling to the bottom does not have time to decompose due to lack of oxygen for decomposers
2) as a result, a thick layer of silt forms at the bottom, which increases the level of the bottom and is a good soil for some plants
3) as a result of the growth of such plants, the amount of non-rotting organic matter at the bottom grows even more and the reservoir becomes swamped

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