Dam removal continues to garner attention as a potential river restoration tool. The increasing possibility of dam removal through the FERC relicensing process, as well as through federal and state agency actions, makes a critical examination of the ecological benefits and costs essential. This paper reviews the possible ecological impacts of dam removal using various case studies. Restoration of an unregulated flow regime has resulted in increased biotic diversity through the enhancement of preferred spawning grounds or other habitat. By returning riverine conditions and sediment transport to formerly impounded areas, riffle/pool sequences, gravel, and cobble have reappeared, along with increases in biotic diversity. Fish passage has been another benefit of dam removal. However, the disappearance of the reservoir may also affect certain publicly desirable fisheries. 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. Dam removal, although controversial, is an important alternative for river restoration.

When talking about sediment issues at hydropower plants or other hydro facilities operators usually face millions of dollars removing or even reducing their problems. Only when economic and/or ecologic pressure starts to force action companies usually drag the sediment or try to flush it out by the dam’s main valve. A prominent example is the great rinse of the Colorado River in 2008 which is going to be repeated at least every two years, probably more frequently. Here each flush comes with the transport of some hundred thousand tons of sediment, but unfortunately also with the loss of some million cubic meters of water and therefore some million dollars in power generation revenues. Dragging the sediment comes with the shutdown of a concerned facility for several months causing the same or even bigger economical effect.

A hydro storage power plant in Germany almost inoperable due to sedimentation turned out to be the start for a complete different approach. A combination of wet dredging, new equipment operation techniques and plant operation allows for low cost and no negative effect on plant performance. Once implemented, three years later the plant and combined reservoir is going to be free from sediment problems permanently. The new technical approach is transferable on almost any range of plants, small to large and run-of-river to pump storage. It also restores river morphology almost back to its natural situation improving often criticized ecological matters. Best of all: It fully suits European Water Framework Directive and U.S. sediment acts at no extra cost.

Greenhouse gas (GHG) emissions from hydroelectric dams are often portrayed as nonexistent by the hydropower industry and have been largely ignored in global comparisons of different sources of electricity. However, the life cycle assessment (LCA)of any hydroelectric plant shows that GHG emissions occur at different phases of the power plant's life. This work examines the role of decommissioning hydroelectric dams in greenhouse gas emissions. Accumulated sediments in reservoirs contain noticeable levels of carbon, which may be released to the atmosphere upon decommissioning of the dam. The rate of sediment accumulation and the sediment volume for six of the ten largest United States hydroelectric power plants is surveyed. The amount of sediments and the respective carbon content at the moment of dam decommissioning (100 years after construction) was estimated. The released carbon is partitioned into CO2 and CH4 emissions and converted toCO2 equivalent emissions using the global warming potential (GWP) method. The global warming effect (GWE) due to dam decommissioning is normalized to the total electricity
produced over the lifetime of each power plant. The estimated GWE of the power plants range from 128-380 g of CO2eq./kWh when 11% of the total available sediment organic carbon (SOC) is mineralized and between 35 and 104 g of CO2eq./kWh when 3% of the total SOC is mineralized. Though these values are below emission factors for coal power plants (890 g of CO2eq./kWh), the amount of greenhouse gases emitted by the sediments upon dam decommissioning is a notable amount that should not be ignored and must be taken into account when considering construction and relicensing of hydroelectric dams

The flow regime of the gravel-bedded river North Tyne has been regulated by the Kielder reservoir for the past 12 years; for the past nine years, regulation has been dominated by hydropower generation. Diurnal stage fluctuations of up to 0.6m are experienced during periods of peak hydropower flows. The main morphological and sedimentological impacts of this regulation are identified and physical explanations provided for the observed adjustments. The main morphological adjustments are identified as the degradation of riffle spawning grounds, the development of fine ssediment berms along channel margins, the aggradation of pools, vegetation of former gravel shoals and the growth of tributary confluence bars. Sedimentological adjustments are subtle and are characterized by higher percentages of fines within spawning gravels, coarsening of surface gravels and the development of a stable, strong bed fabric. The physical explanations for these adjustments relate to changes in the sediment transport regime controlled by the hydraulics associated with the pool-riffle swquence during hydropower generation

Threats to imperiled freshwater fauna in the U.S. were assessed through an experts survey addressing anthropogenic stressors and their sources. Specifically, cause of historic declines and current limits to recovery were identified for 135 imperiled freshwater species of fishes, crayfishes, dragonflies and damselflies, mussels, and amphibians. The survey was designed to identify threats with sufficient specificity to inform resource managers and regulators faced with translating information about predominant biological threats into specific, responsive actions. The findings point to altered sediment loads and nutrient inputs from agricultural nonpoint pollution; interference from exotic species; and altered hydrologic regimes associated with impoundment operations as the three leading threats nationwide, accompanied by many lesser but still significant threats. Variations in threats among regions and among taxa were also evident. Eastern species are most commonly affected by altered sediment loads from agricultural activities, whereas exotic species, habitat removal/damage, and altered hydrologic regimes predominate in the West. Altered sediment loading from agricultural activities and exotic species are dominant problems for both eastern mussels and fishes. However, whereas eastern mussels appear to be more severely affected by altered nutrient impacts from hydroelectric impoundments and agricultural runoff. Our findings suggest that control of nonpoint source pollution associated with agriculture activities should be a very high priority for agricultural producers and governmental support programs. Additionally, the large number of hydropower dams in the U.S. subject to federal relicensing in coming years suggests a significant opportunity to restore natural hydrologic regimes in the affected rivers.

The Mississippi River is the hardest working river in America: a central artery for commerce, a stormwater management system for the two-thirds of the nation, the central flyway for 40% of the nation's migratory waterfowl. Each of the river's distinct forms of habitat is disappearing: backwater marshes dominated by emergent plants are filling in or, alternatively, becoming open, lifeless turbid waters; floodplain lakes have filled with silt; aquatic plants are not replaced because perpetually turbid waters block light penetration; sediment buries mussel beds and deepwater pools' islands erode, eliminating mast-producing forests. High water tables undermine floodplain forests, which lack higher ground to replace themselves. Restrictions of fish movement by the dams makes the decline of habitat in a particular pool more significant by blocking fish access to habitats in another pool. These problems are exacerbated by current river uses, and by past and present land uses that have altered basinwide hydrology and accelerated the rate at which sediment enters the river. Sediments and nutrients enter the river at unsustainable rates due to past and present land use practices that increase erosion and eliminate wetlands and stream-side buffers. Commercial and recreational vessels resuspend sediments in the water column, blocking light penetration and contributing to the loss of backwaters. Even in its reduced for, the Upper Mississippi represents the last piece of Midwestern America's Great Rivers, supporting migrating waterfowl, endangered mussel species and the most ancient lineage of fish in North America. Whether this system continues to survive and flourish depends on whether dynamic river forces can be sufficiently restored to make the river system self-sustaining. Preserving and restoring the Upper Mississippi and Illinois rivers requires three types of actions: 1. Recreate dynamic river forces to achieve self-sustaining habitat restoration 2. Minimize the operational impacts of the navigation system 3. Achieve no net increase in sediment by 2010

Water resource developments which deplete the quantity or reduce the range of streamflows usually have a number of unintended effects on the channel downstream, including loss of channel capacity, loss of aquatic and riparian habitat, and channel instabilities. A method for identifying a flow regime sufficient to maintain desired stream characteristics, while permitting significant development, would have great practical value. over the past decade, important advances have been made in our understanding of fluvial processes in gravel-bed streams. Using these advances as a basis, one can outline a method for determining channel maintenance flows for gravel-bed streams typical to the western United States. A common characteristic of gravel-bed streams is that bed particles are transported only about 5-10 percent of the time during the highest flows, and, even then, at a very low rate. Although occasional motion of bed particles begins at a discharge as small as 60 percent of the bankfull value, general motion of the bed surface is exceedingly rare. The proposed method relies on an appropriate bedload transport function and specific reach of discharge in the pre- and post-regulation regimes. Evaluation of possible flow regimes indicates that bankfull channel capacity can be maintained in its pre-regulation condition where as much as 60 percent of the natural flows are diverted.