mudflats

/M AH1 D F L AE1 T S/
noun
  1. 1

    Flat land along a river, lying few feet above normal high water, often consisting of alluvial deposits and naturally fertile.

Synonyms

noun
  1. 1

    A muddy expanse of flat land, especially such land as a river bed exposed at low tide.

Source: https://en.wiktionary.org/wiki/bottomland, https://en.wiktionary.org/wiki/mudflat, https://en.wiktionary.org/wiki/mudflats