Preface Drylands : Holocene Climatic, Geomorphic, and Cultural Change on the Canadian Prairies

This Drylands special issue of Géographie physique et Quaternaire (GpQ) originates from the Great Plains Geomorphology and Environmental Change session, convened at the May 2002 annual meeting of the Geological Association of Canada / Mineralogical Association of Canada (GAC/MAC) in Saskatoon. Presentations at this session represented a convergence of research activities on the Canadian prairies including studies of flood hazards in the Red River basin, historical and Holocene drought on the Canadian prairies, and interdisciplinary geoarcheological investigations contributed by SCAPE (Study of Cultural Adaptations in the Prairie Ecozone) participants. The session was co-sponsored by the Canadian Geomorphology Research Group (CGRG), Geological Survey of Canada (GSC) and the International Geological Correlation Programme project (IGCP-413) on Understanding Future Dryland Changes from Past Dynamics. Although for most Canadians, drylands may bring to mind the arid regions of the Sahel, western Australia, or American southwest, it is the term that accurately describes the Canadian prairies. Technically speaking, drylands include the dry sub-humid, semi-arid and arid regions of the world where the ratio of annual precipitation to evapotranspiration (P: PE) is below 0.65, but not the desert, or hyper-arid, regions of the world where this ratio is below 0.03. Drylands cover nearly 40 % of the total land surface of the Earth. The Canadian prairies share much in common with other drylands of the world; they are a place of modest water supply, with less than that of forest regions but more than that of deserts, characterized by grassland, steppe and parkland, where dry-farming agriculture is an economic activity that dominates rural land use. A common characteristic of dryland regions, where rainfall is typically less than about 500 mm per annum, is the variability in seasonal and annual precipitation. Paradoxically, this variation commonly results in either “too much or too little water”. The first volume in this issue centres on this theme, with papers divided into two sets. The first set of four papers deals with the hydrological reconstruction of river and lake basins on the Canadian prairies, and with flood reconstruction in the Red River and Assiniboine basins. The second set of three papers deals with the reconstruction of episodes of dune activity, and the implications for changing aridity on the Canadian prairies during the Holocene. The first paper, by Last and Teller, attempts to unravel the complex Holocene history of Lake Manitoba, which has been influenced by differential isostatic uplift, variable river and groundwater inflow, and changing climate. The reconstruction of lake level changes through time is assisted by radiocarbon dating, and mineralogical, geochemical and lithostratigraphic analyses. The history of the lake basin begins with isolation from glacial Lake Agassiz at about 8500 BP and re-filling of the basin by 7700 BP, followed by re-routing of inflow from the Assinibone River by 4500 BP and later re-flooding in the late Holocene starting at about 3500 BP. The second paper, by Oetelaar, provides a reconstruction of the evolution of the Bow River Valley during the Holocene. In much the same manner as Last and Teller, the author finds that the history of incision and aggradation has been controlled by changes in rates of isostatic rebound, in base level, and in discharge into the drainage system. This history includes an early interval of aggradation between 11 500 and 10 000 BP resulting from multiple controls, followed by fluvial incision between 10 000 and 9000 BP in response to increased runoff, a second phase of aggradation originating as floodplain overbank deposits between 9000 and 5000 BP in response to increased sediment availability and reduced stream power, and a final phase of downcutting initiated around …