Nile River records indicate very low flow following the 1783--1784 Laki volcanic eruption, as well as after other high-latitude volcanic eruptions. As shown by climate model simulations of the Laki eruption, significant cooling (-1¿ to -3¿C) of the Northern Hemisphere land masses during the boreal summer of 1783 resulted in a strong dynamical effect of weakening the African and Indian monsoon circulations, with precipitation anomalies of -1 to -3 mm/day over the Sahel of Africa, thus producing the low Nile flow. Future high-latitude eruptions would significantly impact the food and water supplies in these areas. Using observations of the flow of the Nile River, this new understanding is used to support a date of 939 for the beginning of the eruption of the Eldgj¿ volcano in Iceland, the largest high-latitude eruption of the past 1500 years.