Assessing the impact of riprap bank stabilization on fish habitat: A study of Lowland and Appalachian streams in Southern Québec

There is a growing concern over the potential environmental impacts of riverbank stabilization using rock riprap as the occurrence of these structures continues to increase in river networks. Habitat diversity and quality are often used as a proxy for fish community health. Habitat assessments, however, frequently yield contrasting results between studies and it remains unclear how non-salmonid species in small streams may be affected by bank stabilization. The aim of this thesis was to evaluate how riprap structures impact fish habitat in small Lowland and Appalachian streams by combining quantitative and qualitative approaches. Metrics measured were: mesohabitat and in-stream cover proportions, Hydro-Morphological Index of Diversity (HMID), and a modified Qualitative Habitat Evaluation Index (QHEI). Results show that in more pristine Appalachian streams, QHEI scores are lower at stabilized reaches due to loss of in-stream cover and riparian vegetation. However, riprap stabilization had less impact on already altered, straightened Lowland streams. In this latter context, some possibly beneficial alterations of fish habitat were observed in riprapped reaches due to the coarsening of the substrate and an induced increase of slope. These positive effects are, however, limited to short stabilized reaches, and extensive (> 100 m) riprapping of the bed should be avoided as it can result in the drying of the bed during summer months, as was observed in this study in some tributaries of the Salvail River. Both metrics (HMID and QHEI) revealed the positive or neutral effect of riprap on increasing flow diversity and heterogeneity for Lowlands sites with a correlation of 0.72 (p