DETERMINING SEDIMENTATION RATES AND PATTERNS OF EDINBORO LAKE

Historic documents suggest that Edinboro Lake (a natural kettle lake in northwest Pennsylvania) has been rapidly in-filling over the last 100 years, based on the notion that the lake was much deeper a few decades ago. According to bathymetric maps from1960 to 1973, the lake has filled in with 4.3 meters of sediment, yielding a sedimentation rate of 33 cm/yr. At that rate, the lake (currently 39 m deep) would fill within 119 years, resulting in a substantial economic and ecological loss. Recent studies also suggest that changes in land-use have increased nutrient loading, resulting in eutrophication and increased sedimentation of the lake since wide-spread development began ca 1960.

To test the hypothesis of rapid sedimentation, and to reconstruct the sedimentation history of Edinboro Lake, a 5.52 m core was extracted (in 8.5m of water) and analyzed for organic carbon (loss on ignition), grain size (laser diffractometer), and clay mineralogy (x-ray diffraction). Three radiocarbon dates constrain the rate of sedimentation. A model of core depth versus age shows a linear (99.9% correlation), slow average sedimentation rate of only 0.56 mm/yr for the last 9540 years.

Analysis of organic carbon, grain size, and clay mineralogy all show only minor variations throughout the core, suggesting that there has been very little change in either the rates or type of sedimentation, despite widespread urbanization of the watershed. In fact, the largest changes in sedimentological properties occur well before urbanization. The largest organic carbon content in the core (corresponding with the poorest lake quality) occurred ca 2270 yr BP. The largest influx of coarse sediment (sand) occurred ca 1200 yr BP, and the largest change in clay mineralogy (decreased proportion of Illite/Smectitie interlayered clays) occurred ca 2970 yr BP.