PALEOECOLOGY OF THE ST. CROIX RIVER: FINDINGS FROM FLOODPLAIN WETLANDS AND LAKE ST. CROIX

Joy M. Ramstack, Croix Watershed Research Station
Mark B. Edlund, Croix Watershed Research Station
Laura D. TriplettCroix Watershed Research Station, University of Minnesota, Department of Geology and Geophysics
Daniel R. Engstrom, University of Minnesota, Department of Geology and Geophysics

The St. Croix River has undergone significant land-use changes since European settlers arrived in the 1840s. The river continues to face major ecological threats as recreational use and development in the watershed increases. For guiding management decisions on the river and the St. Croix National Scenic Riverway, an understanding of the timing and magnitude of change in river conditions before and since European settlement is crucial. Paleolimnology allows us to reconstruct past changes; however, paleolimnology of rivers provides unique challenges. Sediment transport is too episodic and complex to allow accumulation of continuous sedimentary sequences, such as those found in "normal" lakes. Therefore, previous paleolimnological studies on the river have targeted Lake St. Croix. Results of these studies demonstrated that, despite its perception of being a pristine river, significant increases in nutrients, sedimentation rate, and algal productivity have occurred in Lake St. Croix since European settlement. But, environmental impacts are not limited to Lake St. Croix; for that reason, we have analyzed sediment cores from floodplain wetlands as a way of reconstructing ecological conditions on the upper reaches of the St. Croix. Preliminary results from our work on floodplain wetlands, as well as work from Lake St. Croix, demonstrate that dating of sediment cores from riverine systems is extremely challenging. For most cores, careful site selection coupled with a combination of lead-210, cesium-137, and pollen analyses are necessary to provide reliable dating models before analyzing the paleorecord in sediment cores. Once these dating criteria are met, multiple biological and geochemical proxies are analyzed to reconstruct the historical impacts of hydromanagement, flooding, nutrients, landuse, and exotics on the St. Croix floodplain.