The Collaborative International Dictionary of English v.0.48:

Roust \Roust\, n. [Cf. Icel. r["o]st an estuary.]
   A strong tide or current, especially in a narrow channel.
   [Written also rost, and roost.] --Jamieson.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Rost \Rost\, n.
   See Roust. [Scot.] --Jamieson.
   [1913 Webster]