Upstream River Responses to Low-Head Dam Removal

Removal of low-head and mill pond dams throughout North America has garnered considerable attention over the past twenty years. Dam removal along watercourses is frequently being considered to address private and public liability regarding the failure of old and/or abandoned dams, removal of aquatic migration barriers and, to improve nutrient spiraling to enhance both downstream and upstream ecological diversity. A significant number of studies have been conducted over the past 50 years regarding the ecological impacts of dams, rates of reservoir sedimentation and, down stream erosion. Further, several more studies have examined and employed strategies for the decommissioning of dams, limiting downstream sediment deposition and improving fish migration. However, few studies have examined the upstream impacts of dam decommissioning beyond the backwater limits of reservoirs. This study examines the upstream effects of low-head dam decommissioning on channel morphology using the analog of existing failed dams by natural processes. The most drastic method of dam decommissioning is the "blow and go" approach, whereby a dam is removed and the watercourse is simply allowed to reassume a stable morphology (often assumed to be the pre-dam morphology). A series of low-head and mill pond dams were studied in southern Ontario which failed between 6 – 45 years before present and at locations where no upstream channel mitigation has taken place to provide an analog to the "blow and go" methodology. In each study reach, longitudinal profiles and cross-sectional field surveys were conducted from below the dam faces for several meander wave lengths upstream of each reservoir backwater limits. Numerical models were developed for the historical dam operational and current hydraulic conditions. Pavement samples were collected from riffles and headcuts within and above the reservoir regions of each reach and tractive force and sediment transport analyses conducted. Findings demonstrate that erosional disturbances typically in the form of headcuts and lateral migration are frequently observed at considerable distances upstream of reservoir limits. Depending upon the upstream bedload grain size distribution, the rate of channel armouring and dam height (i.e. localized friction slope), channel disturbances may continue for several decades and continue to degrade a given watercourse for several kilometers upstream of a reservoir. Findings further support the need to consider upstream grade-control and floodplain re-connectivity as part of such restoration initiatives to minimize degradational impacts, down stream sedimentation and to stabilize aquatic and riparian habitat.