Integrating hydrogeomorphological concepts in management approaches of lowland agricultural streams in Quebec: Perspectives, problems and prospects.

English Abstract:

River management in agricultural settings has undergone profound changes in recent years, particularly in the Midwest. In particular, 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 of this change. The benefits of these approaches include: decrease of nitrate concentrations in water, better heterogeneity of stream habitat, reduction of field flooding and greater bank stability than the traditional trapezoidal profile typically used in straightened agricultural streams. However, 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 that would allow for some geomorphological adjustments of a small channel comprised within a larger, fixed one. A particular emphasis was put on the subsurface drainage outlets which often limit the degree of adjustment of these small streams. These impacts are analyzed by hydrodynamic modeling (HEC-RAS) and a hydraulic geometry approach. A crop yield analysis in the fluvial corridor along streams prone to frequent flooding was also conducted. Initial findings confirm the improvement of surface drainage for high magnitude floods, but also identify a potentially endemic problem for 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 also explored. One approach would be to create small cavities along the streams that would provide increased heterogeneity and limit the need to dredge agricultural streams over long distances.