Investigating the role of connectivity and scale in assessing the sources of sediment in an agricultural watershed in the Canadian prairies using sediment source fingerprinting

Sediments adversely impact the quality of surface waters and are a significant source of contaminants, such as nutrients and pesticides, in agricultural watersheds. The development of effective beneficial management practices (BMPs) to minimize these impacts requires a sound understanding of the sources of sediments. The objectives of this study were: (1) to determine the sources of sediment in an agricultural watershed in the Canadian prairies using sediment source fingerprinting and; (2) to assess the results of the sediment fingerprinting study within the context of the scale of observation and the hydro-geomorphic connectivity of the watershed. Geochemical and radionuclide fingerprints were used to discriminate between the three potential sediment sources identified: topsoil, streambanks, and shale bedrock. Suspended and bed sediment samples were collected over the course of 3 years at six locations along the main stem of the creek, ranging from 3rd- (48 ha) to 7th-order (7441 ha) drainage basins. Four sediment fingerprint properties were selected that met statistical- and process-based selection criteria and the Stable Isotope Analysis in R model was used to estimate the proportion of sediment derived from each source at each sampling location in the watershed. The suspended sediments in the upper reaches were dominated by topsoil sources (64%-85%), whereas the suspended sediments moving through the lower reaches and being exported from the watershed had a higher proportion of sediment coming from streambank (32%-51%) and shale bedrock (29%-40%) sources. The switch in the sources of sediment between the headwaters and the watershed outlet are due to: (1) changes in sediment storage and connectivity; (2) a transition in the dominant erosion processes from topsoil to streambank erosion; and (3) the incision of the stream through the shale bedrock as it crosses the Manitoba Escarpment. The results of this sediment fingerprinting study demonstrated that there was a switch in sediment sources between the headwaters and the outlet of the watershed. This research highlights the importance of the sampling location, in relation to the scale and geomorphic connectivity of the watershed, on the interpretation of results derived from the sediment fingerprinting technique, particularly in terms of developing suitable watershed BMPs to protect surface waters.