Source : Webster's Revised Unabridged Dictionary (1913)

Overarch \O`ver*arch"\, v. t. & i.
   To make or place an arch over; to hang over like an arch.
   ``Brown with o'erarching shades.'' --Pope.

Source : WordNet®

overarch
     v 1: be central or dominant; "This scene overarches the entire
          first act"
     2: form an arch over; "Big rocks overarch the stream" [syn: {arch
        over}]