Per the Maryland Geological Survey: Q1: Is it true that Maryland does not have any natural lakes? A1: Yes, there are no natural lakes in Maryland. All of Maryland’s lakes are manmade by damming rivers. Some have been named lakes (e.g., Lake Habeeb in Allegany County and Deep Creek Lake in Garrett County), but most have been named reservoirs (e.g., Loch Raven Reservoir in Baltimore County).
Q2: Did Maryland ever have any natural lakes in the past? A2: Yes. We know of at least one, and there could be more. The one clearly documented case is Buckel’s Bog, which was a 160-acre, shallow periglacial lake (actually a glade) that occupied the headwater region of the North Branch of the Casselman River in Garrett County during the late Pleistocene (19,000-14,000 years ago). [Reference: Maxwell, J.A. and Davis, M. B., 1972, Pollen evidence of Pleistocene and Holocene vegetation of the Allegheny Plateau, Maryland: Quaternary Research, 2(4): 506-530.]