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The H218O/H216O ratio of meteoric water in eastern North America decreases with increasing latitude. The annual weighted average δ18O content of the Middle Atlantic Bight (MAB) and Gulf of Maine (GOM) rivers are −9.33‰ and −10.89‰, respectively. The δ18O compositions of the major east coast rivers are 2‰ enriched during summer months. By using H218O and salinity measurements, both conservative properties of sea water, the geographic origin of continental shelf water can be identified. The isotope-salinity tracer method, which has been used so successfully in polar regions, is also an effective method in the temperate MAB because subpolar waters are exported to this region. The apparent dilutent of slope water is −22‰ δ18O relative to standard mean ocean water. Water of this composition is presently forming in the Labrador Sea. Melt water from sea ice has nearly the same isotopic composition as the sea water from which it formed and thus may be distinguished from meteoric water. In March 1977, the New York Bight cold pool contained as much as 2.5% sea ice melt water. The Gulf of St. Lawrence is the nearest source of sea ice. The New York Bight ‘cold pool’ was renewed from the north between March and July, which was the extent of our sampling period.