The Collaborative International Dictionary of English v.0.48:

Slough \Slough\, n. [OE. slogh, slough, AS. sl[=o]h a hollow
   place; cf. MHG. sl[=u]ch an abyss, gullet, G. schlucken to
   swallow; also Gael. & Ir. sloc a pit, pool. ditch, Ir. slug
   to swallow. Gr. ????? to hiccough, to sob.]
   1. A place of deep mud or mire; a hole full of mire.
      --Chaucer.
      [1913 Webster]

            He's here stuck in a slough.          --Milton.
      [1913 Webster]

   2. [Pronounced sl[=oo].] A wet place; a swale; a side channel
      or inlet from a river.

   Note: [In this sense local or provincial; also spelt sloo,
         and slue.]
         [1913 Webster]

   Slough grass (Bot.), a name in the Mississippi valley for
      grasses of the genus Muhlenbergia; -- called also drop
      seed, and nimble Will.
      [1913 Webster]

WordNet (r) 3.0 (2006):

slough grass
    n 1: North American cordgrass having leaves with dry membranous
         margins and glumes with long awns [syn: prairie
         cordgrass, freshwater cordgrass, slough grass,
         Spartina pectinmata]