Perhaps its could be termed a peninsula?
A piece of land almost surrounded by water or projecting out into a body of water.
— Oxford Dictionaries
Wikipedia mentions peninsula used for the land where a river which courses through a very tight meander which seems similar to OP's requirement. Assuming here the term can be used regardless for the surrounding waters' direction of flow.
A peninsula (Latin: paeninsula from paene "almost" and insula "island") is a piece of land extending out into a body of water that is still connected to mainland, or a piece of land that is bordered by water on three sides but connected to mainland. [...] The surrounding water is usually understood to be continuous, though not necessarily named as a single body of water. Peninsulas are not always named as such; one can also be a headland, cape, island promontory, bill, point, or spit. [...] A river which courses through a very tight meander is also sometimes said to form a "peninsula" within the (almost closed) loop of water.