Both climatic and isostatic changes have affected lake levels in Maine. If climate change were the sole cause of level change, lake-level fluctuations were uniform around a lake. If isostasy were the sole cause, lake-levels were higher on one side of the lake than the other. In this study we tested the hypothesis that glacial loading and isostatic depression of the earth’s crust affected Rangeley Lake's level as well as climatatic changes. Rangeley Lake is located in western Maine at an elevation of 460 m. The lake is rectangular, with an area of 25 km2 and an inlet to the southeast and an outlet to the northwest. The lake averages about 29 m deep in a central basin, with shallow margins all around. The lake bottom was completely mosaic-mapped with side scan sonar, seismic reflection and bathymetric profiles. Ground-penetrating radar was employed to study lake margins, and cores were collected from near the lake inlet. An historic investigation was made of the impact of humans on the lake through dam construction and removal. Mud was mapped in most of the lake; sand and bedrock were uncommon. Gravel marks the occurrence of moraines in the southwestern part of the lake. From seismic observations we interpret a bedrock basement with patchy till deposits up to 10 m thick. A well laminated, acoustically transparent unit, interpreted as glacial-lacustrine mud occurs throughout the lake basin over till or bedrock and averages about 5 m thick. Modern, acoustically very transparent gyttja covers much of the lake except in the southwest margin shallower than 15 m depth. Above this depth gyttja is thin to absent and glacial-lacustrine reflectors are truncated by erosion. A prominent sigmoidal bathymetric shape at 15 m is interpreted as a shoreline deposit. The absence of this transgressive stratigraphy on the north side of the lake suggests predominant isostatic influence on lake levels, with tilting down-to-northwest in late-glacial time and uplift toward the present.