The Collaborative International Dictionary of English v.0.48:

Dalles \Dalles\ (d[a^]lz), n. pl. [F. dalle a tube, gutter,
   trough.]
   A rapid, esp. one where the channel is narrowed between rock
   walls. [Northwestern U. S. & Canada]

         The place below, where the compressed river wound like
         a silver thread among the flat black rocks, was the
         far-famed Dalles of the Columbia.        --F. H. Balch.
   [Webster 1913 Suppl.]