From mud + flat.
mudflat (plural mudflats)
- A muddy expanse of flat land, especially such land as a river bed exposed at low tide.
1899 February, Joseph Conrad, “The Heart of Darkness”, in Blackwood’s Edinburgh Magazine, volume CLXV, number M, New York, N.Y.: The Leonard Scott Publishing Company, […], OCLC 1042815524, part I, page 194:
The Chapman lighthouse, a three-legged thing erect on a mud-flat, shone strongly.
flat expanse of mud, esp. tidal flat