The Collaborative International Dictionary of English v.0.48:

Milldam \Mill"dam`\ (m[i^]l"d[a^]m), n.
   A dam or mound to obstruct a water course, and raise the
   water to a height sufficient to turn a mill wheel.
   [1913 Webster]

WordNet (r) 3.0 (2006):

milldam
    n 1: dam to make a millpond to provide power for a water mill