Answer:
they are expanding tidal mouths of rivers
Usually defined as where a river enters the sea, but a similar feature may be found much less commonly, on a lake shore.
An Estuary (estuarine as an adjective) is formed where a small (even tiny) river enters a body of water that has an ebb and flow. At ocean edge this ebb and flow is provided by the tides, but on a lake shore, the process is much slower and relies on the seasonal rise and fall of the lake. Particularly if the lake has little sediment.
An estuarine environment may be an erosion environment, if the supply of silt down the river is less than that removed in the drainage phase of water movement. If the supply of silt is great, then coastal lagoons may be formed, though these will depend on silt transport along the coast as well.
Estuaries are a particularly rich ecosystem with their shallow warm waters, and adequate supply of water and nutrient.