Measurements of ΔD and some Δ180 measurements were made on 36 rivers which drain the east coast of the United States. The data are consistent with the work of Friedman et al.(1964). The streamflow data of Bue (1970) were used to estimate the mean isotopic composition of runoff entering the Gulf of Maine, the Mid-Atlantic Bight, and the South Atlantic Bight. Assuming binary mixing between central Atlantic water and these fresh runoff waters, coastal water in the Gulf of Maine is isotopically indistinguishable from runoff into the Mid-Atlantic Bight and runoff from the St. Lawrence at salinities greater than 32%0. However, up to salinities of 34%0 the mean isotopic composition of runoff into the South Atlantic Bight is sufficiently different from the Mid-Atlantic Bight that the identification of coastal water masses in and around Cape Hatteras by ΔD and/or Δ180 versus salinity plots should be possible.