A whole-basin stratigraphic record of sediment and phosphorus loading to the St. Croix River, USA

摘要: Lake St. Croix is a natural impoundment of the lowermost 37 km River in Minnesota and Wisconsin, making this one few large river systems world possessing long-term depositional basin at its terminus. The river’s relatively pristine condition led to designation as National Scenic Riverway 1968, but increasing urbanization lower reaches has raised concerns about impacts on water quality. This study was initiated reconstruct historical loadings suspended sediment phosphorus (P) from record Croix. Twenty-four piston cores, with an average length 2 m, were collected along eight transects lake. Dated chronologies 210Pb, 137Cs 14C used calculate rate accumulation lake over past 100+ years. Diatom microfossil analysis lakewater P concentrations same time period, quantified amount trapped sediments. Using whole-lake mass balance approach, loading last years calculated. Beginning 1850, increased dramatically peak 1950–1960 times background rates prior European settlement. driven largely by contributions small side-valley catchment tributaries downstream half total load sharply after 1940 remains high, around four level pre-European settlement conditions. timing shows that early activities, such logging conversion forest prairie agricultural land between 1850 1890, had only modest By contrast, mid-1900s brought major increases lake, suggesting recent activities landscape changes nutrient balances watershed have caused current eutrophic important recreational resource.