Gomphus (Gomphurus) lynnae Paulson, known from only a few localities in the states of Washington and Oregon, U.S.A., is reported from 10 localities in a 260 km stretch of the John Day River in Gilliam, Grant, Sherman, and Wheeler counties and a 59 km stretch of the Owyhee River in Malheur County, Oregon, all within the Columbia River watershed. We collected adults, nymphs and exuviae from several localities and two nymphs were associated with adults. Nymphs of this rare species have not been fully characterized. The nymph is described and illustrated; its closest congener is G. externus Hagen, from which it is distinguished by the lack of a distinct postero-lateral spine on abdominal segment 6, shorter postero-lateral spine on abdominal segment 9, and the presence of only two or three denticles, sometimes lacking, on the lateral margins of abdominal segment 8. Nymphs of G. lynnae occupy substrates of mixed sand and silt in riffle/run areas of large, slow flowing rivers.