The Collaborative International Dictionary of English v.0.48:

Freshet \Fresh"et\, n. [OE. fresche flood + -et. See Fresh,
   a.]
   1. A stream of fresh water. [Obs.] --Milton.
      [1913 Webster]

   2. A flood or overflowing of a stream caused by heavy rains
      or melted snow; a sudden inundation.
      [1913 Webster]

            Cracked the sky, as ice in rivers
            When the freshet is at highest.       --Longfellow.
      [1913 Webster]

WordNet (r) 3.0 (2006):

freshet
    n 1: the occurrence of a water flow resulting from sudden rain
         or melting snow [syn: freshet, spate]