Martin W. Doyle

Trading off: the ecological effects 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 acrimony arises from our limited scientific knowledge of the effects of dam removal. We believe that the ecological consequences are best understood by viewing the removal process as a disturbance. Ecological outcomes will include changes that are both environmentally costly, such as invasion of exotic species, and environmentally beneficial, such as increasing access to spawning habitats for migratory fish. It has also become apparent that the wholesale aging of the US dam infrastructure will make dam removal even more common in the future. The challenge ahead is to better understand and manage the consequences of these removals.

Stream restoration strategies for reducing river nitrogen loads

Despite decades of work on implementing best management practices to reduce the movement of excess nitrogen (N) to aquatic ecosystems, the amount of N in streams and rivers remains high in many watersheds. Stream restoration has become increasingly popular, yet efforts to quantify N-removal benefits are only just beginning. Natural resource managers are asking scientists to provide advice for reducing the downstream flux of N. Here, we propose a framework for prioritizing restoration sites that involves identifying where potential N loads are large due to sizeable sources and efficient delivery to streams, and when the majority of N is exported. Small streams (1st–3rd order) with considerable loads delivered during low to moderate flows offer the greatest opportunities for N removal. We suggest approaches that increase in-stream carbon availability, contact between the water and benthos, and connections between streams and adjacent terrestrial environments. Because of uncertainties concerning the magnitud...

Short-term changes in channel form and macroinvertebrate communities following low-head dam removal

Although >70 dams have been decommissioned in Wisconsin over the past 30 y, little is known about the physical and ecological effects of dam removal on riverine ecosystems. The purpose of our study was to document changes in channel form and macroinvertebrate assemblages following the removal of a low-head, run-of-river dam from the Baraboo River, Wisconsin, in January 2000. We surveyed cross sections and collected benthic macroinvertebrate samples in 6 reaches (an upstream reference reach, reaches immediately above and below the dam that was to be removed, and sequential unimpounded and impounded reaches further downstream) in a multiple-dam system. Surveys were conducted in December 1999, before dam removal, in March 2000 immediately after dam removal, and in August 2000 following a flood. Benthic sediments were collected from selected sites in March and August to measure particle size shifts associated with the dam breach. Before dam removal, impounded reaches were characterized by relatively deep, wide channels, extensive deposits of loose sediments, and macroinvertebrate taxa characteristic of lentic or depositional habitats. Removal of the dam significantly decreased the cross-sectional area of the active channel in the former impoundment from 59 m2 to 11 m2, but did not alter channel form in downstream reaches. However, we found an increase in loose sediments and in the relative abundance of the sand fraction (0.5-2.0 mm) below the dam immediately after it had been removed. A flood in June increased cross-sectional area in the former impoundment by widening the channel. Sediments that had settled in the reaches below the dam in March were transported ∼3.5 km downstream, where they became trapped in another impoundment. The flood had little or no detectable effect on the other 5 study reaches. Within 1 y of dam removal, macroinvertebrate assemblages in formerly impounded reaches did not significantly differ from those in either the upstream reference site or in other unimpounded reaches below the dam site. All unimpounded sites were characterized by lotic taxa such as net-spinning caddisflies and heptageniid mayflies regardless of their impoundment history. Thus, dam removal caused relatively small and transient geomorphic and ecological changes in downstream reaches, and apparently rapid channel development to an equilibrium form within the impoundment, associated with both dam removal and the subsequent June flood. These muted changes and rapid recovery likely result from the relatively large channel size and the small volume of stored sediment available for transport following dam removal.

Channel adjustments following two dam removals in Wisconsin

[1] We examined channel response following the removal of low-head dams on two lowgradient, fine- to coarse-grained rivers in southern Wisconsin. Following removal, channels eroded large quantities of fine sediment, resulting in deposition 3–5 km downstream. At one site (Baraboo River), upstream changes were rapid and included bed degradation, minimal bank erosion, and sediment deposition on channel margins and new floodplain. Sand was transported through the former impoundment and temporarily deposited downstream. At the second site (Koshkonong River), head-cut migration governed channel adjustments. A deep, narrow channel formed downstream of the headcut, with negligible changes upstream of the head-cut. Fluvial changes were summarized in a conceptual channel evolution model that highlighted (1) similarities between adjustments associated with dam removal and other events that lower channel base-level, and (2) the role of reservoir sediment characteristics (particle size, cohesion) in controlling the rates and mechanisms of sediment movement and channel adjustment. INDEX TERMS: 1815 Hydrology: Erosion and sedimentation; 1824 Hydrology: Geomorphology (1625); 1857 Hydrology: Reservoirs (surface); KEYWORDS: dams, dam removal, channel evolution model

Effective discharge analysis of ecological processes in streams

[1] Discharge is a master variable that controls many processes in stream ecosystems. However, there is uncertainty of which discharges are most important for driving particular ecological processes and thus how flow regime may influence entire stream ecosystems. Here the analytical method of effective discharge from fluvial geomorphology is used to analyze the interaction between frequency and magnitude of discharge events that drive organic matter transport, algal growth, nutrient retention, macroinvertebrate disturbance, and habitat availability. We quantify the ecological effective discharge using a synthesis of previously published studies and modeling from a range of study sites. An analytical expression is then developed for a particular case of ecological effective discharge and is used to explore how effective discharge varies within variable hydrologic regimes. Our results suggest that a range of discharges is important for different ecological processes in an individual stream. Discharges are not equally important; instead, effective discharge values exist that correspond to near modal flows and moderate floods for the variable sets examined. We suggest four types of ecological response to discharge variability: discharge as a transport mechanism, regulator of habitat, process modulator, and disturbance. Effective discharge analysis will perform well when there is a unique, essentially instantaneous relationship between discharge and an ecological process and poorly when effects of discharge are delayed or confounded by legacy effects. Despite some limitations the conceptual and analytical utility of the effective discharge analysis allows exploring general questions about how hydrologic variability influences various ecological processes in streams.

Dam removal in the United States: Emerging needs for science and policy

The aging of Americas dams, coupled with increasing awareness of their environmental costs, has brought dam decommissioning and removal to the attention of the scientific community, management agencies, and the general public. Over the past two years, dam removal has been the focus of special sessions at the annual meetings of numerous scientific societies (e.g., American Association for the Advancement of Science, American Geophysical Union, Ecological Society of America, Association of American Geographers) as well as groups focusedon bridging science and policy [Heinz Center, 2002]. Here we briefly examine why dam removal has emerged as a critical issue, the realities of ongoing dam removal efforts and the current policy vacuum, and the need for expanded scientific research to support policy development.

Aging Infrastructure and Ecosystem Restoration

Targeted decommissioning of deteriorated and obsolete infrastructure can provide opportunities for restoring degraded ecosystems.

Privatizing stream restoration in the US

In this paper, we use a case study of the stream restoration field to demonstrate how the particular state and market logics of neoliberalism are shifting both the practice of restoration scientists and the relations between public and private sector science. In particular, the embrace of neoliberal environmental management regimes has intensified the demand for environmental scientists to produce applied science that can: (1) be taught as a standardized package; (2) be used by agencies to justify decisions; and (3) form the basis for new markets in ecosystems services. At this point, private sector science produces the most influential knowledge claims, the most widely used applications, and the primary educational system for stream restoration in the US. We argue that the needs of markets and regulatory agencies are heavily implicated in this privatization process, and that the resulting impacts on restoration science and the dynamics of the stream restoration field in the US thus cannot be described without attention to political—economic relations.

Response of Unionid Mussels to Dam Removal in Koshkonong Creek, Wisconsin (USA)

Dam removal is a potentially powerful tool for restoring riverine habitats and communities. However, the effectiveness of this tool is unknown because published data on the effects of dam removal on in-stream biota are lacking. We investigated the effects of a small dam removal on unionid mussels in Koshkonong Creek, Wisconsin (USA). Removal of the dam led to mortality both within the former impoundment and in downstream reaches. Within the former reservoir, mortality rates were extremely high (95%) due to desiccation and exposure. Mussel densities in a bed 0.5km downstream from the dam declined from 3.80±0.56 musselsm−2 in fall 2000 immediately after dam removal to 2.60±0.48 musselsm−2 by summer 2003. One rare species, Quadrula pustulosa, was lost from community. Mortality of mussels buried in deposited silt was also observed at a site 1.7km below the dam. Silt and sand increased from 16.8 and 1.1% of total area sampled in fall 2000 to 30.4 and 15.9%, respectively, in summer 2003. Total suspended sediment concentrations in the water column were always higher downstream from the reservoir than upstream, suggesting that transport and deposition of reservoir sediments likely contributed to downstream mussel mortality. Thus, while benefits of the dam removal included fish passage and restoration of lotic habitats in the former millpond, these changes were brought about at some cost to the local mussel community. Pre-removal assessments of potential ecological impacts of dam removal and appropriate mitigation efforts should be included in the dam removal process to reduce short-term negative ecological effects of this restoration action.

Stacking ecosystem services

Ecosystem service markets are increasingly used as a policy solution to environmental problems ranging from endangered species to climate change. Such markets trade in ecosystem credits created at restoration sites where conservation projects are designed and built to compensate for regulated environmental impacts. “Credit stacking” occurs when multiple, spatially overlapping credits representing different ecosystem services are sold separately to compensate for different impacts. Discussion of stacking has grown rapidly over the past three years, and it will generate increasing interest given the growing multibillion-dollar international market in carbon, habitat, and water-quality credits. Because ecosystem functions at compensation sites are interdependent and integrated, stacking may result in net environmental losses. Unless stacked compensation sites and impact sites are treated symmetrically in the accounting of environmental gains and losses, stacking may also cause environmental gains at compensa...