The Collaborative International Dictionary of English v.0.48:

bottomland \bottomland\ n.
   low-lying alluvial land near a river.

   Syn: bottom.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

bottomland
    n 1: low-lying alluvial land near a river [syn: bottomland,
         bottom]

Moby Thesaurus II by Grady Ward, 1.0:

91 Moby Thesaurus words for "bottomland":
   alkali flat, alluvial plain, basin, baygall, bog, bottom, bottoms,
   buffalo wallow, bushveld, campo, champaign, champaign country,
   coastal plain, delta, desert, down, downs, everglade, fell, fen,
   fenland, flat, flat country, flatland, flats, glade, grass veld,
   grassland, heath, hog wallow, holm, lande, level, llano, lowland,
   lowlands, lunar mare, marais, mare, marish, marsh, marshland,
   meadow, mere, mesa, mesilla, mire, moor, moorland, morass, moss,
   mud, mud flat, open country, pampa, pampas, peat bog, peneplain,
   plain, plains, plateau, playa, prairie, quagmire, quicksand,
   salt flat, salt marsh, salt pan, savanna, sebkha, slob land,
   slough, sough, steppe, sump, swale, swamp, swampland, table,
   tableland, taiga, tree veld, tundra, upland, vega, veld, wallow,
   wash, weald, wide-open spaces, wold