mud flat

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun Low-lying muddy land that is covered at high tide and exposed at low tide.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A flat expanse of mud at the edge of a body of water, regularly inundated by e.g. tidal action.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a tract of low muddy land near an estuary; covered at high tide and exposed at low tide