What is Backwater?

Legal Definition
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.
-- Bouviers Law Dictionary
Legal Definition
Water in a stream which, in consequence of some dam or obstruction below, is detained or checked in its course, or flows back. Hodges v. Raymond, 9 Mass. 316 ; Chambers v. Kyle, 87 Ind. 85. Water caused to flow backward from a steam-vessel by reason of the action of its wheels or screw.
-- Black's Law Dictionary
Legal Definition
Water backed up by obstruction of a stream.
-- Ballentine's Law Dictionary