Abstract

River management in agricultural settings has undergone profound changes in recent years, particularly in the Midwest. A better integration of hydrogeomorphological principles has led to novel approaches with the two-stage channel design for small agricultural streams being among the best examples. The applicability of these innovative techniques in Quebec remains to be tested. We conducted detailed analyses of streams in agricultural areas located in Montérégie (Quebec) to determine the impacts of a change in the trapezoidal channel profile. A particular emphasis was put on the subsurface drainage outlets which often limit the degree of adjustment of these small streams. These impacts were analyzed by hydrodynamic modeling (HEC-RAS) and a hydraulic geometry approach. Findings confirm the improvement of surface drainage for high magnitude floods, but also identify a potentially endemic problem in Quebec where, due to the rectangular shape of agricultural fields (an inheritance from French settlements), very deep subsurface drainage outlets are in conflict with natural floodplain generation within the straightened channels. Alternative measures to accommodate for natural fluvial adjustments while maintaining drainage efficiency were explored. One potential approach would be to create small pocket wetlands along the streams that would provide increased heterogeneity and limit the need to dredge agricultural streams over long distances. Lastly, crop yields within fluvial corridors were greatly influenced by flood connectivity, with markedly reduced yields in near-stream zones. This indicates that acquisition costs for critical riparian land could be greatly diminished for many of the critical areas needed for hydrogeomorphological processes to operate.