The Collaborative International Dictionary of English v.0.48:

Backwater \Back"wa`ter\, n. [Back, a. or adv. + -water. ]
   1. Water turned back in its course by an obstruction, an
      opposing current, or the flow of the tide, as in a sewer
      or river channel, or across a river bar.
      [1913 Webster]

   2. An accumulation of water overflowing the low lands, caused
      by an obstruction.
      [1913 Webster]

   3. Water thrown back by the turning of a waterwheel, or by
      the paddle wheels of a steamer.
      [1913 Webster]

WordNet (r) 3.0 (2006):

backwater
    n 1: a body of water that was created by a flood or tide or by
         being held or forced back by a dam; "the bayous and
         backwaters are breeding grounds for mosquitos"
    2: a place or condition in which no development or progress is
       occurring; "the country is an economic backwater"

Moby Thesaurus II by Grady Ward, 1.0:

108 Moby Thesaurus words for "backwater":
   Charybdis, Maelstrom, abjure, arrest, back, back away, back down,
   back off, back out, back stream, back up, backcountry, backflow,
   backpedal, backtrack, backtrail, backwash, backwoods, boondocks,
   brake, bush, check, climb down, clip the wings, countercurrent,
   counterflow, counterflux, countermarch, crawfish out, cry off,
   curb, decelerate, delay, deny, detain, disavow, disclaim, disown,
   dodge, draw rein, duck, ease off, ease up, eat crow,
   eat humble pie, eddy, elude, evade, forswear, get around,
   go astern, go into reverse, gulf, gurge, hinterland, hold back,
   hold in check, hold up, impede, keep back, let down, let up,
   lose ground, lose momentum, lose speed, maelstrom, make sternway,
   moderate, obstruct, recant, reef, refluence, reflux, regurgitation,
   rein in, relax, renege, renounce, repudiate, resile, retard,
   retract, reverse, revoke, set back, shirk, sidestep, slack off,
   slack up, slacken, slow, slow down, slow up, stay, sticks, swallow,
   swirl, take back, take in sail, throttle down, twirl, unsay,
   up-country, vortex, welsh, whirl, whirlpool, withdraw



Bouvier's Law Dictionary, Revised 6th Ed (1856):

BACK-WATER. That water in a stream which, in consequence of some obstruction
below, is detained or checked in its course, or reflows.
     2. Every riparian owner is entitled to the benefit of the water in its
natural state. Whenever, therefore, the owner of land dams or impedes the
water in such a manner as to back it on his neighbor above, he is liable to
an action; for no one has a right to alter the level of the water, either
where it enters, or where it leaves his property. 9 Co. 59; 1 B. & Ald. 258;
1 Wils. R. 178; 6 East, R. 203; 1 S. & Stu. 190.; 4 Day, R. 244; 7 Cowen, R.
266; 1 Rawle, R. 218; 5 N. R. Rep. 232; 9 Mass. R. 316; 7 Pick. R. 198; 4
Mason, R. 400; 1 Rawle, R. 27; 2 John. Ch. R. 162, 463; 1 Coxe's. R. 460.
Vide, Dam; Inundation; Water-course; and 5 Ohio R. 322.