ASSESSING THE SOURCES OF SUSPENDED SEDIMENTS IN THE STREAMS OF AN AGRICULTURAL WATERSHED IN THE PRAIRIES USING 137Cs AS A TRACER

Sediments adversely impact the quality of surface waters and are a significant source of contaminants such as nutrients and pesticides in agricultural watersheds. South Tobacco Creek watershed, located in south central Manitoba, is a watershed of national significance in the study of agriculture's impacts on water quality. This predominantly agricultural watershed extends across the Manitoba Escarpment; its upper reaches lay in undulating glacial tills and its lower reaches lay in the lacustrine sediments of glacial Lake Agassiz. Past studies of soil erosion within fields and sediment delivery from the watershed have produced conflicting results. In 2009, a comprehensive study of the sources of sediments was undertaken using sediment fingerprinting techniques. Suspended sediments were sampled using a time-integrated sampler fixed to the stream bed. Samples were collected over the course of two years at five locations along the main stem of the creek, ranging from 3rd order (42 ha) to seventh order (7441 ha) drainage basins. Sediment samples were analyzed for Cs-137 content and these values were compared to those measured within soil and streambank profiles. Analysis indicated that the majority of suspended sediments being exported from the watershed were coming from the stream channels and not the soils of the uplands.