Geohydrologic setting of Mirror Lake, West Thornton, New Hampshire

The drainage basin of Mirror Lake, New Hampshire is characterized by high knobs and ridges and steep land slopes. The lake is situated in the lower part of the basin, largely within glacial drift. Drift in most of the Mirror Lake drainage basin is till, which is as much as 50 meters thick, Crystalline bedrock underlying the drift is composed of schist, slate, and quartz monzonite of earliest Devonian age. These rocks are intensely folded and contain numerous fractures. Three small streams flow into Mirror Lake. Streamflow discharge into Mirror Lake differs between the two largest subbasins. Basin NW, which has a more round shape and thicker glacial drift than basin W, has greater stream discharge as well as greater sustained base flow. Ground water moves into most parts of Mirror Lake, and losses to ground water occur principally on the southeast side. Water-level data from piezometers and bedrock wells indicate dynamic ground-water movement within the fractured bedrock underlying Mirror Lake. These data also indicate very active interchange of ground water between bedrock and the overlying glacial drift. There are particularly strong hydraulic head gradients from the bedrock into the drift in the area between Mirror Lake and Hubbard Brook. (USGS)