from The Century Dictionary and Cyclopedia
- n. In a canal or river, the body of water at any part of its course as determined by the cross-section at that part, regarded as a cross-section of a prism.
Sorry, no etymologies found.
By setting a water-prism -- water contained in a wedge-shaped vessel with glass sides (B, fig. 8) -- in opposition to a wedge of glass (to the right of B), this point can be illustrated before you.