The Collaborative International Dictionary of English v.0.48:

Strath \Strath\, n. [Gael. srath.]
   A valley of considerable size, through which a river runs; a
   valley bottom; -- often used in composition with the name of
   the river; as, Strath Spey, Strathdon, Strathmore. [Scot.]
   [1913 Webster]

         The long green strath of Napa valley.    --R. L.
                                                  Stevenson.
   [1913 Webster]