Removal of in-channel structures

Effectiveness for Biological Quality Elements

Because dam and weir removal is a relatively new technique, there is not extensive published literature on its effectiveness (Roni et al., 2005).

In considering the effectiveness of dam and weir removal, it is necessary to consider both short-term impacts (e.g. sediment release) and longer-term gains. Stanley et al. (2001) examined the short-term changes in channel form and macroinvertebrate communities following low-head dam removal. Within one year of remavl macroinverterbrate assemblages in the former impounded reaches did not differ significantly from those in the upstream reference sites or downstream. As only a small amount of sediment was released as a result of dam removal downstream effects on macroinvertebrates were minimal. A limited number of 'erosion check' weirs, identified as pinch points, were removed on watercourses in Denmark in the 1980's, with apparent immediate success at restoring continuity for fish (unpublished, cited in Jacobs 2008).

Orr et al. (2008) examined the influence of a pair of small dam removals on downstream periphyton and macroinvertebrates in Boulder Creek, WI (USA). The dams were 180m apart and both were removed on 9 July 2003. After the removals, an initial release of sediment significantly increased the proportion of fine sediments in the downstream reach and butted benthic substrate. This burial led to a 60% reduction in chlorophyll in the downstream reach the week following removal, while concentrations increased in the upstream reference reach. Similarly, macroinvertebrate densities two weeks post-removal were lower relative to pre-removal densities and were associated with declines of ephemeropterans, trichopterans and dipterans. Examination of Trichoptera genera demonstrated substantial changes in this assemblage associated with diminshed densities of the formerly dominant genus Brachycentrus. Algal and invertebrate populations increased in the weeks after the dam removal, but did not reach densities similar to the upstream reference reach. In the following year, both periphyton and invertebrate densities were lower in the downstream reach, suggesting a long-term effect of the removal. Thus, effects of the dam removal were alternatively positive or negative over time, and indicate that the time scale of consideration has a strong effect on the interpretation of the consequences of this management activity.

Stanley et al (2007) examined fish community composition two years prior to and two years after the removal of a pair of low-head dams from Boulder Creek WI (USA) in 2003 to determine if removal of these potential barriers affected the resident population of native brook trout (Salvelinus fontinalis). Despite the presence of other taxa in the downstream reaches, and in other similar streams adjacent to the Boulder Creek (including the brown trout, Salmo trutta), no new species had colonised the Boulder Creek in the two years following dam removal.

Short-term ecological impacts of dam removal include an increased sediment load that may cause suffocation and abrasion to various biota and habitats. However, several recorded dam removals have suggested that the increased sediment load caused by removal should be a short-term effect. Pre-removal studies for contaminated sediment may be effective at controlling toxic release problems. Although monitoring and dam removal studies are limited, a continued examination of the possible ecological impacts is important for quantifying the resistance and resilience of aquatic ecosystems.

In the UK, monitoring of Salmon and Trout prior to and following removal of obsolete barriers has indicated that this technique has been effective in facilitating fish migration, providing access to a greater proportion of the river network. Case stude examples include removal of a dam at the tidal limit of the Water of App in South Ayrshire as part of the Tolka Flood Relief Scheme.

References and suggested further reading

Roni, P., Hanson, K., Beechie, T., Pess, G., Pollock, M. and Bartley, D. (2005) Habitat rehabilitation for inland fisheries: global review of effectiveness and guidance for rehabilitation of freshwater ecosystems. Food and Agriculture Organization of the United Nations, Rome, 2005. Section 2.4: Dam Removal and Flood Restoration