The Collaborative International Dictionary of English v.0.48:

Spatter-dock \Spat`ter-dock`\, n. (Bot.)
   The common yellow water lily (Nuphar advena).
   [1913 Webster]

WordNet (r) 3.0 (2006):

Nuphar advena
    n 1: common water lily of eastern and central North America,
         having broad leaves and globe-shaped yellow flowers; in
         sluggish fresh or slightly brackish water [syn:
         spatterdock, cow lily, yellow pond lily, Nuphar
         advena]