Sediment hiatuses, detritus layers, and inwashed terrestrial moss layers in five cores at Crawford Lake, Ontario, Canada, show that the lake level was low between ca. 4.8 and 2 ka (1 ka = 1000 14C yr B.P.). This low level is attributed to a dry and warm climate, which has also been documented at other sites in southern Ontario, southern Michigan, and southern Wisconsin. Oxygen isotope (δ18O) values from authigenic marl show a negative shift of 2.4‰ from −9‰ to −11.4‰ (Vienna Peedee belemnite) between ca. 5 and 2 ka. Enhanced evaporation under dry and likely closed-basin conditions would lead to more enrichment in 18O, so we suggest that the trend to depleted 18O indicates a significant change in the δ18O value of the source meteoric water. The major moisture source for the Great Lakes region is the Gulf of Mexico, from which the amount and seasonality of precipitation are affected by the interplay of air masses from the Gulf of Mexico and North Pacific, probably controlled by jet stream positions and storm tracks. In the late middle Holocene, the isotopically heavy moisture from the gulf might have contributed less precipitation and/or a higher proportion in winter months, probably caused by more frequent eastward extension of dry Pacific air depleted in 18O. This hypothesis implies that the δ18O values of paleoprecipitation in the middle Holocene reflected moisture-source history more strongly than paleotemperature.