Landscape Evolution at Wanuskewin Heritage Park, Saskatoon Saskatchewan.

Regional deglaciation of the study site (520 13' N, 1060 35' W) and drainage of glacial Lake Saskatchewan occurred by ca. 10.5 ka BP. Channelisation of flow into the South Saskatchewan River and a gradual drop in base level caused small tributary stream valleys, such as the Opimihaw Creek valley, to be incised through surficial sediments and underlying till formations. Large stream discharges associated with a cool, moist postglacial climate contributed to the erosion of the landscape. The following physiographic elements were observed in the study area: the till plain; alluvial terraces; mass movement landforms; the modem stream channel. The alluvial terraces located in the valley bottom are the focus of this study. Prior to ca. 4.6 ka BP, a combination of a rise in base level and gradual climate change led to the crossing of a geomorphic threshold, shifting the stream from an incising to an aggrading system. Slope wash resulting from a reduction in vegetation cover, combined with variable precipitation and stream discharge, led to initial rapid channel aggradation. Sedimentation rates gradually declined as the environment became increasingly moist leading to a more constant stream discharge, a denser vegetation cover and reduced slope wash. Excavations at several valley bottom alluvial terraces reveal five sedimentary facies: facies 1, vertical accretion sediments; facies 2, proximal channel sediments; facies 3, alluvium and/or colluvium; facies 4, fluvial sands; facies 5, channel gravels. The generally fining upwards sequence from coarse-grain fluvial channel deposits (facies 3 - 5) to finer-grained proximal channel and vertical accretion sediments (facies 1 and 2) records the migration of the stream channel across its floodplain throughout the aggradation phase. Repeated pedogenesis and human occupation indicate periodic subaerial exposure of the floodplain. Downstream sites are generally dominated by finergrained facies and are characterised by more rapid sedimentation rates than upstream sites. Within the last ca O. 1 to 0.2 ka BP, a geomorphic threshold was crossed and the stream incised its floodplain. The proximity of the South Saskatchewan river to alluvial terraces at the mouth of the creek indicates the creek was likely responding to a drop in base level. Currently, there is very little flow in the creek, due in part to several beaver dams along its length