system
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human
[ELI5] Why do ponds and lakes not become swamps after years of season changes

Lakes and Ponds are water reservoirs that are often surrounded by trees, bushes and other plants. During a summer period a lot of "trash" comes into this water, during the fall time plants loose their leaves that often go down to the water surface and later drown. Leaves are being decomposed and every year this cycle repeats. 

Why do not all ponds and lakes change into swamps? Obviously some of them with time transform into bogs but majority stays OK.
gpt
When the leaves decompose, the carbohydrates that make up most of their bulk turn back into water (which joins the lake's volume and eventually outflows through whatever outlet the lake has) and carbon dioxide (which outgasses into the atmosphere). The same happens for other organic molecules. What's left is mostly minerals gathered by the plants surrounding the lake, which (on the time-scales we're talking about) is essentially just plant-driven erosion. The only time this doesn't happen is if the plant material can't effectively decompose, typically because the water is too acidic or too low on oxygen to allow microbial growth (which is how you get many of the wetlands that actually exist in the world).

Many of the remaining minerals (particularly earth metals like sodium) are highly water-soluble, so the rate at which they outflow scales with how much is currently in the lake (which is why lakes that have an outflow are, almost without exception, freshwater). That leaves a few minerals that don't dissolve very well even over many years. Over geological timescales, this stuff does build up. As with any other erosion, highlands wear down and lowlands fill up, and eventually your lake at the bottom of a valley becomes part of a marshy plain full of silt. But this takes a long time, and other forces (primarily plate tectonics) are acting to push some parts of the land back up, too, in a way that can (and sometimes does) prevent this process from reaching its endpoint.