The term RIVER MOUTH FEATURE is a crossword solution that can be filled in with the word DELTA, which is a five-letter word. A DELTA is a landform created by sediment deposits at the mouth of a river. It is typically triangular in shape and is formed when the river deposits sediment as it enters a larger body of water such as a lake or ocean. Deltas are important ecosystems that provide habitat for many species of plants and animals.