Improving fish habitat productivity through controlled dam removals and fishway construction as part of a reclamation plan

Abstract

Mining sites are frequently equipped with dams and reservoirs for water supply. At the time of closure, the dam decommissioning represents an important component of reclamation programmes. Several small dams in North America are reaching the end of their lifespan and the removal process is becoming an increasingly popular technique in the restoration of fish habitats. During the reclamation process at the Heath Steele Mine (NB, Canada), approximately 90 km2 of drainage basin area has been reopened to fish migration in the North Branch of the Miramichi River system through dam removals and modifications to existing obstructions. It was difficult to attribute any changes in the water temperature regime to the dam removal because data were only available after the dam was removed. Nonetheless, no major impact on water temperature was detected as a result of the dam removal. An increase in water temperature was observed in only one branch of the newly formed stream (draining within the old reservoir). Water temperature in other branch remained cold (and similar to its upstream site), suggesting the presence of groundwater seepage. The mean monthly water temperature increased by a maximum of 2.2oC at the site located downstream of the decommissioned dam. However, water temperature at the mouth of the Little South Branch Tomogonops River (10.5 km downstream of the decommissioned dam) returned to values similar to those recorded at the inlet of the old reservoir. This decrease in water temperature can be attributed to the forest canopy (stream shading), groundwater seepage or colder tributaries flowing into the main stem in this section of the river. Brook trout abundance in the newly formed stream within the reservoir section was low (<0.04 individual/m2) compared to the downstream site (<1.2 individual/m2) but was within densities frequently observed in other streams of the province of New Brunswick. Another dam removal project was conducted in White Rapids Brook (Miramichi River) in 2006. Although sediment transport was present at some sites following the removal, the impacts were low, localised, and of short duration. Atlantic salmon recolonised the newly accessible habitat within one year of the dam removal. The planned work sequence and proper mitigative techniques were key components in protecting fish habitat in these projects. Moreover, results showed that when proper techniques were implemented, negative impacts were minor and localised compared to the long-term benefits. These projects represented a unique opportunity for researchers and habitat managers to study the recovery of fish habitat following dam removals, and to better understand river restoration processes. Such studies dealing with river restoration will permit the development of procedures, methods and guidelines for decision-making in future dam removal projects.