Erica+Brown's+Wiki

Dam Removal  **__The benefits of dam removal: __** Most dams in Michigan were built decades ago and many have deteriorated due to age, erosion, poor maintenance, flood damage, and poor designs. Those dams that no longer make sense, that stand in disrepair, or are not removed are at significant risk of failure, particularly during high flow events. Dams that no longer serve a purpose should be removed for safety, economic, and environmental reasons. Furthermore, good proportions of aging dams are no longer economically practical or cost-effective to operate. Similarly, dam operation and maintenance costs tend to increase as a dam ages. These increased costs, combined with the potentially lower revenue, allow for removal to become the most cost-effective alternative for the dam owner. Also, dam removal eliminates the expenses of future maintenance and repairs, and provides several ecological benefits. For example, dam removal restores the natural flowing character of a stream and restores essential ecological processes in the river. Large segments of previously inaccessible water may be open to use by a variety of fish species. In addition, dam removal and sediment management can restore buried fish spawning habitat and other critical stream habitat. Overall, dam removal benefits riverine fish by: (1) removing obstructions to upstream and downstream migration; (2) restoring natural riverine habitat; (3) restoring natural seasonal flow variations; (4) eliminating siltation of spawning and feeding habitat above the dam; (5) allowing debris, small rocks and nutrients to pass below the dam, creating healthy habitat; (6) eliminating unnatural temperature variations below the dam; and (7) removing turbines that kill fish. With regard to resource management, the most significant benefits of dam removal include:  re-connection of important seasonal fish habitat, normalized temperature regimes, improved water clarity (in most cases), improved dissolved oxygen concentrations, normalized sediment and energy transport, and improved biological diversity. In general, carp prefer the warm waters of an impoundment, yet when a dam is removed the cool water species such as trout and bass, generally preferred by anglers, can move back into the river and re-populate.   **__The costs of dam removal: __** Dam removal is gaining credibility as a viable management option for dams that have deteriorated physically and are no longer economically practical. However, the decision to remove or repair a dam is often contentious and emotionally charged. Part of the difficulties arises from our limited scientific knowledge of the effects of the dam removal. While there may be several benefits to removing dams, there can also be costs associated with removing dams as well. For example, once a dam is removed, organisms present in the reservoir prior to removal may  be washed downstream or stranded during surface water drawdown. Also, mortality rates of virtually all reservoir populations, except fish, will be extremely high and can be expected to approach 100 percent if dewatering is rapid. Furthermore, a chief concern for agencies responsible for removing a dam is the management of sediments within the reservoir. Dam removal can result in decades of accumulated material being released downstream in a rapid and catastrophic fashion. Unfortunately, despite awareness of the importance of sediment management, there is remarkable uncertainty regarding patterns and rates of sediment transport following dam removal. Another concern of removing a dam is that often much of the nutrient-rich sediment represents prime habitat for invasion of weedy and exotic species that are generally considered undesirable. One prime example of another problem that can occur with dam removal is the case of the removal of the Fort Edwards Dam on the Hudson River in New York State. Following partial removal of the Fort Edwards Dam in 1973, large quantities of oils and sediments rich in polychlorinated biphenyls (PCBs) were released into the river, requiring a costly cleanup effort. The sediment moved into the river, where it restricted flow and blocked the navigation channel and access to adjacent riverside businesses. The altered flow created an additional health hazard when sewage, discharged into the river by the town of Fort Edwards, could not be conveyed downstream. A second wave of contaminated sediments was mobilized in 1991, when the remaining structure was removed. The following year, average PCB concentrations in striped bass had doubled.

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