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polder
It is an area of land that is drained by a river. It is also a ridge or elevated area of land that divides two areas of land that are drained by two river systems.
watershed
watershed
A river
A catchment.
Land drained by a river is a watershed. This is an area of land that feeds all the water running under it and draining off of it into a body of water.
river
watershed
I believe you might be referring to a "watershed".
jjj
Island is the name that best describes a piece of land sarrounded by water