The Collaborative International Dictionary of English v.0.48:

Flashboard \Flash"board`\, n.
   A board placed temporarily upon a milldam, to raise the water
   in the pond above its usual level; a flushboard. [U.S.]
   [1913 Webster]

WordNet (r) 3.0 (2006):

flashboard
    n 1: boarding place along the top of a dam to increase its
         height [syn: flashboard, flashboarding]