The isotopic composition of precipitation and river runoff in the vicinity of the North American Great Lakes is characterized by a higher deuterium-excess value than observed in the advecting air masses. It is suggested that this indicates that evaporated moisture from the surface waters is mixed with the atmosphere waters. A preliminary estimate of the atmospheric water balance during summer and autumn indicates that between 4.6%--15.7% of the atmospheric water content downwind from the Great Lakes is derived from lake evaporation during summer. ¿ American Geophysical Union 1994