Topographic Fluctuations Across a Spring Fen and Raised Bog in the Lost River Peatland, Northern Minnesota

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Abstract
(1) A topographic survey was conducted with both laser and electronic levels across a spring fen, a raised bog and the water track (fen) between them, in the Lost River Peatland of northern Minnesota. (2) The survey demonstrated that the spring fen consisted of a peat mound whose slopes were steeper than that of the bog. (3) Three successive surveys in 1982 and 1983 showed altitudinal changes in benchmarks fixed in trees relative to a base level on a power-line pylon located in the water track. Nearly all benchmarks rose, with the greatest rises occurring near the centres of the raised bogs (11 cm) and spring fen (6 cm) in contrast to very slight rises across the water track. (4) The most likely explantation for this altitudinal change is a swelling of the sub-surface peat in response to artesian fluid pressure generated by regional hydraulic gradients. (5) Very precise determinations of altitude are thus possible with a laser of electronic level on an unstable peat substrate, providing great care is taken in surveying techniques.