LATE HOLOCENE HISTORY OF PRE- AND POST-SETTLEMENT ENVIRONMENTAL CHANGE IN FLOODPLAIN LAKES OF THE RED RIVER (MANITOBA AND NORTH DAKOTA)

The late Holocene paleoecology of three lake basins (Horseshoe Lake, Lake Louise and Salt Lake) located on the floodplain of the Red River (Manitoba and North Dakota) was reconstructed using microfossil remains (pollen, diatoms and thecamoebians), sedimentology and bulk geochemistry. Microfloral and microfaunal assemblages were used to interpret anthropogenically-induced changes to the limnology and the landscape surrounding the lake basins. Two zones were identified on the basis of pollen chronology, interestingly diatom and thecamoebian assemblages conform to these zones. In Zone II , representing the pre-settlement period prior to AD ~1800, prairie herbs and Quercus pollen dominate and characterize a tall grass prairie and oak riparian forest in the surrounding Red River landscape. Diatom and thecamoebian assemblages indicate that all three lake basins were oligotrophic during this period and that Horseshoe Lake waters were more brackish than at present. The start of Zone I corresponds with changes to the landscape imposed by the large-scale arrival of European settlers to the region. A sharp decline in Quercus, which occurred from the base of this zone, reflects riparian deforestation and the widespread increase in weeds, such as Salsola, Brassica, Rumex and Ambrosia, and cereal grasses associated with the introduction of European agricultural practices. Diatom and thecamoebian assemblages suggest a progressive eutrophication of the floodplain lakes, as well as a general increase in salinity at Salt Lake, through to present day. The frequency and magnitude of Red River floods is partly controlled by climatic factors such as winter/spring precipitation and the timing of spring thaw throughout the basin. Sedimentation in the lake basins is episodic and is dominated by allochotonous material deposited during large-scale flood events, mostly occurring as late-spring ice-jam floods. Paleoflood deposits are indistinguishable from background sedimentation as they comprise virtually the entire sediment package at each lake.