Rivers everywhere have been broken into pieces by dams, weirs, culverts, and bridges. This segmentation is likely to increase, posing significant challenges for river managers and conservationists. Two recent papers discuss some subtle, pervasive challenges to riparian connectivity. Januchowski-Hartley and colleagues show the problems of inventorying and assessing dams and road crossings, and Agostinho and colleagues point out the perils of piscivory to fish ascending ladders.
Januchowski-Hartley et al. use a variety of databases to lay out the network of dams in North America’s Great Lakes region. However, these databases do not include road crossings, which often have comparable impacts to dams and are far more numerous. In aggregate, road crossings and dams make rivers into gauntlets: in each of the four study areas detailed in the paper, passage was obstructed at least 42% of the time. One area only had unobstructed passage 1% of the time. Fortunately, managers across the United States have begun to turn their attention to road crossings that may be improved with less expense and acrimony than dams. The important conservation point is to include these barriers in assessments of basin-scale restoration, and to assess their impact along with dams’. Culverts and half-open passageways still cause problems for riparian biota, as shown by Agostinho et al. in their work on the Tocantins River.
Brazil’s immense Lajeado Dam allows fish to pass upstream through a fish ladder. But unsurprisingly, this results in greater concentrations of fish under the ladder as well as in the ladder mechanism itself. The upshot is that predators are attracted to the dam as well – the ladder amounts to an all-you-can-eat buffet for piranhas (Serrasalmus rhombeus), dolphins (Inia geoffrensis) and other predators. Such problems are commonplace elsewhere: in many places fishing is banned in reaches immediately downstream of dams, and on the Columbia River, managers have to contend with California sea lions (Zalophus californianus), which swim up to the Bonneville Dam (river km 235) to gorge themselves into obesity on salmon (Milstein 2008).
Both of these studies underline the importance of attention to detail and monitoring in river restoration. Partial passage barriers are very difficult to assess without thorough and consistent observation. It is especially important to set baselines, to creatively manage problems like ladder predation, and to measure changes after barrier modification or removal, the better to inform future decisions. Unfortunately, such monitoring is inconsistently funded. But as increasing demands are put on rivers, the sort of close analysis indicated in these papers will be necessary for optimal ecological management.
Resources
Agostinho A., C. Agostinho, F. Pelicice and E. Marques. 2012. Fish ladders: safe fish passage or hotspot for predation? Neotropical Ichthyology 10(4):687-696.
American Rivers (2013) 63 dams removed to restore rivers in 2012.
Januchowski-Hartley, S., P. McIntyre, M. Diebel, P. Doran, D. Infante, C. Joseph, and J. Allan. 2013. Restoring aquatic ecosystem connectivity requires expanding inventories of both dams and road crossings. Front Ecol Environ; doi:10.1890/120168.
