There are no natural lakes in Maryland
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.]
Q3: Why are there no natural lakes in Maryland?
A3: There are about a dozen major types of lakes, meaning there are about a dozen ways lakes form. None of those is found in Maryland. Some 74% of all lakes are glacial in origin, but glaciers never entered Maryland during the last Great Ice Age. Glacial lakes may form in bedrock depressions gouged out by glaciers or in areas where detached blocks of stagnant or retreating ice sheets are surrounded by other glacial deposits, such as sand and gravel outwash. When the blocks of ice melt away, the remaining depression, known as a kettle, may fill with water to form a “kettle lake.” Other major types of natural lakes include those that result from faulting, volcanic activity, and landslides blocking a river.
http://www.mgs.md.gov/geology/maryland_lakes_and_reservoirs.html