WordNet (r) 3.0 (2006):

cwm
    n 1: a steep-walled semicircular basin in a mountain; may
         contain a lake [syn: cirque, corrie, cwm]

Moby Thesaurus II by Grady Ward, 1.0:

73 Moby Thesaurus words for "cwm":
   abysm, abyss, arroyo, box canyon, breach, break, canyon, cavity,
   chap, chasm, check, chimney, chink, cirque, cleft, cleuch, clough,
   col, combe, corrie, coulee, couloir, crack, cranny, crevasse,
   crevice, cut, defile, dell, dike, ditch, donga, draw, excavation,
   fault, fissure, flaw, flume, fracture, furrow, gap, gape, gash,
   gorge, groove, gulch, gulf, gully, hole, incision, joint, kloof,
   leak, moat, notch, nullah, opening, pass, passage, ravine, rent,
   rift, rime, rupture, scissure, seam, slit, slot, split, trench,
   valley, void, wadi