Assessment of Upstream and Downstream Passability for Eel at Dams

The American eel has experienced a marked decline in Canada. Habitat loss resulting from dam construction is one of the factors involved. There are some 5600 dams in rivers draining to the St. Lawrence River system. Their passability for eels migrating upstream and downstream has been assessed using the Québec national dams database. Although the majority of the dams are less than 3 m in height and are made of concrete or earthfill, dams present a great variety of physical characteristics. Passability ranks were assigned to each category of dam based on three assessment criteria: the height of the dam, the materials used in its construction, and its use. Passability to upstream migrants was also assessed from photographs for a subset of dams. The two methods (statistical analysis and the use of photographs) may yield different results, but the two methods were consistent to identify the impassable dams. This analysis shows overall that the problem of passability is more significant for upstream passage than it is for downstream passage. Once added to a geospatial analysis tool which locates dams and calculates the surface area of habitats lost in each watershed, passability ranks will assist managers in setting priorities for mitigation.