Search for a paleoflood record using physical and geochemical properties of cores from shallow lakes, Red River valley, Manitoba and North Dakota

The physical and geochemical characteristics of three lakes in the Red River valley reveal a strong anthropogenic influence on the limnological systems. Agricultural nitrogen and phosphorus input has turned Lake Louise and Horseshoe Lake into eutrophic water bodies with brackish water and high algal production during the growing season. The consumption of oxygen results in dysoxic to anoxic lake bottoms and may account for the large quantities of organic matter preserved within the sediment. An anoxic sediment column has resulted in metal cycling and the obliteration of any flood layers deposited in the lake basins. In contrast, Salt Lake is a well oxygenated, eutrophic saline lake. Water samples reveal distinctive and different chemical signatures for each of the lakes and for the Red River; however, chemical shifts resulting from lake-basin inundation are not recorded in the lake-sediment cores. Page 1 Resume : Les caracteristiques physiques et geochimiques de trois lacs dans la vallee de la riviere Rouge revelent une forte influence anthropique recente sur les systemes limnologiques. Des apports d’azote et de phosphore de sources agricoles ont transforme les lacs Louise et Horseshoe en des plans d’eau eutrophiques aux eaux saumâtres et a forte production d’algues au cours de la saison de croissance. La consommation d’oxygene fait en sorte que les fonds des lacs sont dysoxiques a anoxiques et elle pourrait etre la cause des grandes quantites de matiere organique preservees dans les sediments. Une colonne sedimentaire anoxique a entraine un recyclage des metaux et l’occultation de toute couche de depot d’inondation qui se serait mise en place a l’interieur des bassins lacustres. Par opposition, le lac Salt est un lac sale eutrophique, bien oxygene. Les echantillons d’eau revelent des signatures chimiques caracteristiques et differentes d’un lac a l’autre et des lacs par rapport a la riviere Rouge. Toutefois, les carottes de sediments extraites du fond des lacs ne presente pas la trace de deplacements chimiques qui resulteraient d’inondations dans les bassins lacustres. INTRODUCTION Following the 1997 flood disaster, the Red River Valley Flood Protection Program was created to enhance flood protection and flood management in southern Manitoba (see Topping and Caligiuri, 1999). As part of this program, the Geological Survey of Canada and the Manitoba Geological Survey are investigating the paleoflood history and geological controls on flooding for the Red River (Brooks et al., 1999; St. George et al., 1999). This paper deals with one particular aspect of the study, the examination of lacustrine sediments in the Red River vicinity as a proxy to reconstruct a pre–nineteenth century paleoflood record and a paleoenvironmental framework for the Red River valley. Southern Manitoba is prone to large-scale, ice-jam flooding during the spring freshet of the Red River. In 1997, the area experienced the third largest flood event since 1826. After the floodwaters receded, a thick mud drape resulting from the flood could be found throughout the flooded Red River valley. Floodplain lakes that had experienced inundation were cored and a distinct 1.5 to 3.0 cm thick floodbed was observed. As a result of these observed flood deposits, a research plan was formulated to look for flood layers in floodplain lake cores. Floodplain lakes are natural basins located proximal to the river and receiving river input only during large flood episodes. Riverine sediments and water chemistry are markedly different from the normal lake waters and background lacustrine sedimentation. In this paper, lithological (grain size and mineralogical), geochemical, and chronological data from lake-sediment cores from Horseshoe Lake and Lake Louise in Manitoba, and Salt Lake in North Dakota are discussed. These results are compared and contrasted with data from the Red River, and the data are assessed to determine if a flood record can be reconstructed. This information will supplement biostratigraphic work that is in progress and will be summarized in a subsequent publication. The cores represent the late Holocene history of sedimentation in all three lake basins and complement an existing pollen history from Lake Louise (Brooks and Grenier, 2001). STUDY AREA The Red River is a low-energy, suspended-load meandering river that migrates laterally at a relatively low rate. Reflecting this, there are only eight channel-scar and oxbow lakes between the Canada–United States border and Lake Winnipeg. Two perennial lakes, Lake Louise and Horseshoe Lake (Fig. 1), were chosen for coring. Although it is not a channel-scar lake and lies outside the hydrological floodplain of the Red River, Salt Lake in North Dakota (Fig. 1) was also chosen for coring. It is a saline lake that is subject to backflooding by fresh water when the Red River is in flood. The change in water salinities caused by periods of backflooding offered a rare opportunity to look for a geochemical flood signature.