Undamming Rivers: A Review of the Ecological Impacts of Dam Removal

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. Preremoval 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.     

A Decade of Geomorphic and Hydraulic Response to the La Valle Dam Project,Baraboo River,Wisconsin

We investigate stream response to the La Valle Dam removal and channel reconstruction by estimating channel hydraulic parameter values and changes in sedimentation within the reservoir. The designed channel reconstruction after the dam removal included placement of a riffle structure at the former dam site. Stream surveys undertaken in 1984 by Federal Emergency Management Agency and in 2001 by Doyle et al. were supplemented with surveys in 2009 and 2011 to study the effects of the instream structure. We created a model in HEC‐RAS IV and surface maps in Surfer© using the 1984, 2009, and 2011 surveys. The HEC‐RAS IV model for 2009 channel conditions indicates that the riffle structure decreases upstream channel shear stress and velocity, causing renewed deposition of sediment within the former reservoir. We estimate by 2009, 61% of former reservoir sediments were removed during dam removal and channel reconstruction. Between 2009 and 2011 renewed sedimentation within the former reservoir represented approximately 7.85% of the original reservoir volume. The HEC‐RAS IV models show the largest impacts of the dam and riffle structure occur at flood magnitudes at or below bankfull. Thus, the riffle and the dam similarly alter channel hydraulics and sediment transport. As such, our models indicate that the La Valle Dam project was a dam replacement rather than a removal. Our results confirm that channel reconstruction method can alter channel hydraulics, geomorphology, and sediment mobility.     

Channel change,sediment transport,and fish habitat in a coastal stream: Effects of an extreme event

A study on sediment transport and channel change was conducted on Zayante Creek and the lower San Lorenzo River in Santa Cruz County, California. A rainstorm with a recurrence interval locally in excess of 150 years occurred during the study year, 1982 WY. Stream surveys indicated that significant aggradation occurred during and after the peak flood. Upper study reaches were substantially recovered after high flows of early April, but the lower study reaches still had significant filling of pools and burial of riffles by sand. Increases in width-depth ratio were minor and localized in upper reaches, but were significant in lower reaches. Large inputs of sand, primarily from landsliding, altered the sediment transport regime. A higher proportion of the bedload is now transported by lower flows than before the January event. Roads and sand quarries contributed significantly to sediment input to the stream. A proposed dam may alter the sediment transport regime of Zayante Creek. Mitigating the effects of this dam on downstream fish habitat may require occasional bankfull discharges.     

ASSESSMENT USING GIS AND SEDIMENT ROUTING OF THE PROPOSED REMOVAL OF BALLVILLE DAM,SANDUSKY RIVER,OHIO1

ABSTRACT: The proposed removal of Ballville Dam was assessed by (1) using a new Geographic Information Systems (GIS) based method for calculating reservoir sediment storage, (2) evaluating sediment properties and contamination from core data, and (3) assessing downstream impacts from sediment routing calculations. A 1903 (pre‐dam) map was manipulated using GIS to recreate the reservoir bathymetry at time of dam construction and used in combination with a detailed 1993 bathymetric survey to calculate sediment volumes and thickness. Reservoir sediment properties and geochemistry were determined from 14 sediment vibracores. Annual sedimentation rates varied from 1.7 to 4.3 g/cm²/yr based on Cesium‐137 (¹³⁷Cs) and Lead‐210 (²¹⁰Pb) geochronology and dated flood layers. The pore fluid geochemistry (Ba, Co, Cu, Mn) of four cores showed surficial enrichments in Cu, while Co and Mn show secondary peaks within the sediments. GIS calculations showed that a designed channel through the former reservoir able to accommodate the 10 percent Probable Maximum Flood (PMF) would require removing approximately 0.35 million m³ of sediment (27 percent of the reservoir fill), either by dredging at a cost of up to $6.3 million or by releasing fine grained sediment downstream. A sediment routing model was applied for the critical 6 km downstream using four cross sections. The sediment routing model predicts that, for flows exceeding minimum Mean Daily Flow (1924 to 1998 data), greater than 90 percent of this sediment would be transported through downstream reaches into Lake Erie (Sandusky Bay).     

GEOMORPHIC ANALOGIES FOR ASSESSING PROBABLE CHANNEL RESPONSE TO DAM REMOVAL1

ABSTRACT: There is a pressing need for tools to predict the rates, magnitudes, and mechanisms by which sediment is removed from a reservoir following dam removal, as well as for tools to predict where this sediment will be deposited downstream and how it will impact downstream channel morphology. In the absence of adequate empirical data, a good initial approach is to examine the impacts of dam removal within the context of the geomorphic analogies of channel evolution models and sediment waves. Channel changes at two dam breaching sites in Wisconsin involved a succession of channel forms and processes consistent with an existing channel evolution model. Sediment transported downstream after removal of other dams suggests that reservoir sediment may be translated downstream either as a distinct wave or gradually eroded away. More extensive data collection on existing dam removals is warranted before undertaking the removal of a large number of dams. However, if removal is to proceed based on current knowledge, then geomorphic analogies can be used as the foundation for sediment management and stabilization schemes.     

ASSESSMENT OF METHODS FOR MEASURING EMBEDDEDNESS: APPLICATION TO SEDIMENTATION IN FLOW REGULATED STREAMS1

Abstract: Five commonly used methods for measuring embeddedness the — degree to which fine particles surround coarse substrate on the surface of the streambed — are assessed and used to evaluate the sedimentation pattern resulting from impoundment on tributaries of the Connecticut River. Results show that the U.S. Environmental Protection Agency (USEPA) method best reflects the sediment regime on these rivers. On the Ompompanoosuc River, regulated by a run-of-the-river/flood control dam, embeddedness increases significantly directly downstream of the dam. On the unregulated White River, no downstream trends in embeddedness are observed. The USEPA results on the Ompompanoosuc River reflect the movement of a local decrease in embeddedness, interpreted as a moving region of scour, with a calculated transport rate of approximately 5 to 25 m/day. Observed transport rates are similar to previously measured sediment transport rates and consistent with results from a multifraction sediment transport model. Application of the USEPA method to an additional regulated tributary demonstrates the effects of dam management on embeddedness. Flow regulation with high sediment trapping efficiency results in a decrease in embeddedness downstream of the dam. Results provide insight into the utility of available methods for evaluating the effects of management practice on streambed composition.     

AN INTEGRATIVE APPROACH TOWARDS UNDERSTANDING ECOLOGICAL RESPONSES TO DAM REMOVAL: THE MANATAWNY CREEK STUDY1

ABSTRACT: Dam removal has been proposed as an effective method of river restoration, but few integrative studies have examined ecological responses to the removal of dams. In 1999, we initiated an interdisciplinary study to determine ecological responses to the removal of a 2 m high dam on lower Manatawny Creek in southeastern Pennsylvania. We used an integrative monitoring program to assess the physical, chemical, and biological responses to dam removal. Following removal in 2000, increased sediment transport has led to major changes in channel form in the former impoundment and downstream reaches. Water quality did not change markedly following removal, probably because of the impoundment's short hydraulic residence time (less than two hours at base flow) and infrequent temperature stratification. When the impoundment was converted to a free flowing reach, the composition of the benthic macroinvertebrate and fish assemblages in this portion of Manatawny Creek shifted dramatically from lentic to lotic taxa. Some fish species inhabiting the free flowing reach downstream from the dam were negatively affected by large scale sediment transport and habitat alteration following dam removal, but this appears to be a short term response. Based on our observations and experiences in this study, we provide a list of issues to evaluate when considering future dam removals.     

In situ removal of copper from sediments by a galvanic cell

This study dealt with in situ removal of copper from sediments through an electrokinetic (EK) process driven by a galvanic cell. Iron (Fe) and carbon (C) were placed separately and connected with a conductive wire. Polluted sediments were put between them and water was filled above the sediments. The galvanic cell was thus formed due to the different electrode potentials of Fe and C. The cell could remove the pollutants in the sediments by electromigration and/or electroosmosis. Results showed that a weak voltage less than 1V was formed by the galvanic cell. The voltage decreased with the increase of time. A slight increase of sediment pH from the anode (Fe) to the cathode (C) was observed. The presence of supernatant water inhibited the variation of sediment pH because H(+) and OH(-) could diffuse into the water. The removal of copper was affected by the sediment pH and the distribution of electrolyte in sediment and supernatant water. Lower pH led to higher removal efficiency. More electrolyte in the sediment and/or less electrolyte in the supernatant water favored the removal of copper. The major removal mechanism was proposed on the basis of the desorption of copper from sediment to pore solution and the subsequent electromigration of copper from the anode to the cathode. The diffusion of copper from sediment to supernatant water was negligible.     

AFTER THE 1993 FLOOD: A WATER AND SURFICIAL BED SEDIMENT QUALITY SCENARIO ON THE ILLINOIS AND UPPER MISSISSIPPI RWERS1

ABSTRACT: Effects of the 1993 flood on river water and sediment quality were investigated using historical data and data collected from the Illinois River and Upper Mississippi River in a post‐flood period. Overall the post‐flood results showed systematic reductions and individual changes in the water and sediment constituents. The reductions in sediment metals and nutrients were most obvious at the Keokuk and Lock and Dam 26 stations. By analyzing and comparing the physical changes to the changes in water and sediment constituents at each station, it was found that physical processes such as sediment entrainment and, more importantly, the removal of fine sediment to be the main causes for the reduced concentrations in sediment constituents. On the other hand, sediment redistribution and associated secondary contamination could have caused the emergence of several water and sediment constituents that were undetected before the flood.     

Flood control failure: San Lorenzo River,California

The San Lorenzo River on the central California coast was the site of a major US Army Corps of Engineers flood control project in 1959. By excavating the channel below its natural grade and constructing levees, the capacity of the river was increased in order to contain approximately the 100 year flood. Production and transport of large volumes of sediment from the river's urbanizing watershed has filled the flood control project with sand and silt. The natural gradient has been re-established, and flood protection has been reduced to containment of perhaps the 30 year flood. In order for the City of Santa Cruz, which is situated on the flood plain, to be protected from future flooding,it must either initiate an expensive annual dredging program, or replan and rebuild the inadequately designed flood control channel. It has become clear, here and elsewhere, that the problem of flooding cannot simply be resolved by engineering. Large flood control projects provide a false sense of security and commonly produce unexpected channel changes.     

EFFECTS OF DAM IMPOUNDMENT ON THE FLOOD REGIME OF NATURAL FLOODPLAIN COMMUNITIES IN THE UPPER CONNECTICUT RIVER1

ABSTRACT: Understanding the effects of dams on the inundation regime of natural floodplain communities is critical for effective decision making on dam management or dam removal. To test the implications of hydrologic alteration by dams for floodplain natural communities, we conducted a combined field and modeling study along two reaches in the Connecticut River Rapids Macrosite (CRRM), one of the last remaining flowing water sections of the Upper Connecticut River. We surveyed multiple channel cross sections at both locations and concurrently identified and surveyed the elevations of important natural communities, native species of concern, and nonnative invasive species. Using a hydrologic model, HEC‐RAS, we routed estimated pre‐and post‐impoundment discharges of different design recurrence intervals (two year through 100 year floods) through each reach to establish corresponding reductions in elevation and effective wetted perimeter following post‐dam discharge reductions. By comparing (1) the frequency and duration of flooding of these surfaces before and after impoundment and (2) the total area flooded at different recurrence intervals, our goal was to derive a spatially explicit assessment of hydrologic alteration, directly relevant to natural floodplain communities. Post‐impoundment hydrologic alteration profoundly affected the subsequent inundation regime, and this impact was particularly true of higher floodplain terraces. These riparian communities, which were flooded, on average, every 20 to 100 years pre‐impoundment, were predicted to flood at 100 ? 100 year intervals, essentially isolating them completely from riverine influence. At the pre‐dam five to ten year floodplain elevations, we observed smaller differences in predicted flood frequency but substantial differences in the total area flooded and in the average flood duration. For floodplain forests in the Upper Connecticut River, this alteration by impoundment suggests that even if other stresses facing these communities (human development, invasive exotics) were alleviated, this may not be sufficient to restore intact natural communities. More generally, our approach provides a way to combine site specific variables with long term gage records in assessing the restorative potential of dam removal.     

The Aging of America's Reservoirs: In‐Reservoir and Downstream Physical Changes and Habitat Implications

Reservoirs are important for various purposes including flood control, water supply, power generation, and recreation. The aging of America's reservoirs and progressive loss of water storage capacity resulting from ongoing sedimentation, coupled with increasing societal needs, will cause the social, economic, environmental, and political importance of reservoirs to continually increase. The short‐ and medium‐term (<50 years) environmental consequences of reservoir construction and operation are well known and include an altered flow regime, lost connectivity (longitudinal, floodplain), an altered sediment regime, substrate compositional change, and downstream channel degradation. In general, reservoir‐related changes have had adverse consequences for the natural ecosystem. Longer term (>50 years) environmental changes as reservoirs enter “old” age are less understood. Additional research is needed to help guide the future management of aging reservoir systems and support the difficult decisions that will have to be made. Important research directions include assessment of climate change effects on aging and determination of ecosystem response to ongoing aging and various management actions that may be taken with the intent of minimizing or reversing the physical effects of aging.     

Assessment of spatial variation of cesium-137 in small catchments

Surface contamination by bomb-derived and Chernobyl-derived 137Cs has been subject to changes due to physical decay and lateral transport of contaminated soil particles, which have resulted in an on-going transfer of radionuclides from terrestrial ecosystems to surface water, river bed sediments, and flood plains. Knowledge of the different sources of spatial variation of 137Cs is particularly essential for estimating 137Cs transfer to fluvial systems and for successfully applying 137Cs as an environmental tracer in soil erosion studies. This study combined a straightforward sediment redistribution model and geostatistical interpolation of point samples of 137Cs activities in soil to distinguish the effects of sediment erosion and deposition from other sources of variation in 137Cs in the small Mochovce catchment in Slovakia. These other sources of variation could then be interpreted. Besides erosion and deposition processes, the initial pattern of 137Cs deposition, floodplain sedimentation, and short-range spatial variation were identified as the major sources of spatial variation of the 137Cs inventory.     

Influence of Small Dams on Downstream Channel Characteristics in Pennsylvania and Maryland: Implications for the Long‐Term Geomorphic Effects of Dam Removal1

Abstract: We evaluate the effects of small dams (11 of 15 sites less than 4 m high) on downstream channels at 15 sites in Maryland and Pennsylvania by using a reach upstream of the reservoir at each site to represent the downstream reach before dam construction. A semi‐quantitative geomorphic characterization demonstrates that upstream reaches occupy similar geomorphic settings as downstream reaches. Survey data indicate that dams have had no measurable influence on the water surface slope, width, and the percentages of exposed bedrock or boulders on the streambed. The median grain diameter (D₅₀) is increased slightly by dam construction, but D₅₀ remains within the pebble size class. The percentage of sand and silt and clay on the bed averages about 35% before dam construction, but typically decreases to around 20% after dam construction. The presence of the dam has therefore only influenced the fraction of finer‐grained sediment on the bed, and has not caused other measurable changes in fluvial morphology. The absence of measurable geomorphic change from dam impacts is explicable given the extent of geologic control at these study sites. We speculate that potential changes that could have been induced by dam construction have been resisted by inerodible bedrock, relatively immobile boulders, well‐vegetated and cohesive banks, and low rates of bed material supply and transport. If the dams of our study are removed, we argue that long‐term changes (those that remain after a period of transient adjustment) will be limited to increases in the percentage of sand and silt and clay on the bed. Thus, dam removal in streams similar to those of our study area should not result in significant long‐term geomorphic changes.     

Upstream Channel Changes Following Dam Construction and Removal Using a GIS/Remote Sensing Approach1

Abstract: This study used an innovative GIS/remote sensing approach to study historical river channel changes in the Huron River, a wandering gravel‐bedded river in northern Ohio. Eight sets of historical aerial photographs (1958‐2003) span the construction of a low‐head dam (1969), removal of the spillway (1994), and removal of the dam itself (2002). Construction of the dam modified stream gradients >4 km upstream of the small impounded reservoir. This study tracked changes in the polygon size, shape, and centroid position of 12 sand‐gravel bars through a study reach 0.2‐4.1 km upstream of the dam. These bars were highly responsive, tending to migrate obliquely downstream and toward the outer bank at rates up to 9 m/year. Historical changes in the size and position of the bars can be interpreted as the downstream translation of one or more sediment waves. Prior to dam construction, a sediment wave moved downstream through the study reach. Following construction of the dam, this sediment wave became stationary and degraded in situ by dispersion. The growth of bars throughout the study reach during this time interval resulted in a progressive increase in channel sinuosity. Removal of the spillway rejuvenated downstream translation of a sediment wave through the study reach and was followed by a reduction in channel sinuosity. These results illustrate that important geomorphologic changes can occur upstream of low‐head dams. This may be a neglected area of research about the effects of dams and dam removals.     

A geomorphologist's criticism of the engineering approach to channelization of gravel-bed rivers: case study of the Raba River, Polish Carpathians

River engineers use sediment transport formulas to design regulated channels in which the river's ability to transport bedload would remain in equilibrium with the delivery of materials from upstream. In gravel-bed rivers, a number of factors distort the simple relationship between particle size and hydraulic parameters at the threshold of sediment motion, inherent in the formulas. This may lead to significant errors in predicting the bedload transport rates in such streams and hence to instability of their regulated channels. The failure to recognize a nonstationary river regime may also result in unsuccessful channelization. Rapid channel incision has followed channelization of the main rivers of the Polish Carpathians in the 20th century. A case study of the Raba River shows that incision has resulted from the increase in stream power caused by channelization and the simultaneous reduction in sediment supply due to variations in basin management and a change in flood hydrographs. Calculations of bedload transport in the river by the Meyer-Peter and Müller formula are shown to have resulted in unrealistic estimates, perhaps because the different degree of bed armoring in particular cross-sections was neglected. It would have been possible to avoid improper channelization if the decreasing trend in sediment load of the Carpathian rivers had been recognized on the basis of geomorphological and sedimentological studies. Allowing the rivers to increase their sinuosity, wherever possible without an erosional threat to property and infrastructure, and preventing further in-stream gravel mining are postulated in order to arrest channel incision and reestablish the conditions for water and sediment storage on the floodplains.     

A LAW AND ECONOMICS APPROACH TO RESOLVING RESERVOIR SEDIMENT MANAGEMENT CONFLICTS1

The loss of the world's reservoir capacity to sedimentation can be mitigated by altering dam operations to release sediment downstream. However, legal uncertainty regarding whether dam owners are liable for damages to surrounding landowners due to altered operations provides a significant disincentive for sustainable sediment management. Past work recommends that courts apply a “rule of reasonableness” in assessing liability: dam owners should be held liable for damages only if they act unreasonably in altering operations, and surrounding landowners should take reasonable measures to mitigate foreseeable damage. Based on past cases, judicial determinations of reasonable reservoir management and reasonable precautionary measures by landowners are generally highly speculative, controversial, and based on limited information. Courts can ease the future burden of making these difficult determinations with rulings that create economic incentives for parties to act reasonably. For example, courts might entitle landowners to be free from sediment related damages, and protect the entitlement with a liability rule. This gives dam owners an economic incentive to release sediment only if the benefits of doing so outweigh court ordered damages to landowners. Past judicial decisions are largely consistent with this legal regime.     

Changes in Streambed Composition in Salmonid Spawning Habitat of the Elwha River during Dam Removal

One uncertainty associated with large dam removal is the level of downstream sediment deposition and associated short‐term biological effects, particularly on salmonid spawning habitat. Recent studies report downstream sediment deposition following dam removal is influenced by proximity to the source and river transport capacity. The impacts of dam removal sediment releases are difficult to generalize due to the relatively small number of dam removals completed, the variation in release strategies, and the physical nature of systems. Changes to sediment deposition and associated streambed composition in the Elwha River, Washington State, were monitored prior to (2010‐2011) and during (2012‐2014) the simultaneous removal of two large dams (32 and 64 m). Changes in the surface layer substrate composition during dam removal varied by year and channel type. Riffles in floodplain channels downstream of the dams fined and remained sand dominated throughout the study period, and exceeded levels known to be detrimental to incubating salmonids. Mainstem riffles tended to fine to gravel, but appear to be trending toward cobble after the majority of the sediment was released and transported through system. Thus, salmonid spawning habitats in the mainstem appear to have been minimally impacted while those in floodplain channels appear to have been severely impacted during dam removal.     

Assessing potential bioavailability of metals in sediments: A proposed approach

Due to anthropogenic inputs, elevated concentrations of metals frequently occur in aquatic sediments. In order to make defensible estimates of the potential risk of metals in sediments and/or develop sediment quality criteria for metals, it is essential to identify that fraction of the total metal in the sediments that is bioavailable. Studies with a variety of benthic invertebrates indicate that interstitial (pore) water concentrations of metals correspond very well with the bioavailability of metals in test sediments. Many factors may influence pore water concentrations of metals; however, in anaerobic sediments a key phase controlling partitioning of several cationic metals (cadmium, nickel, lead, zinc, copper) into pore water is acid volatile sulfide (AVS). In this paper, we present an overview of the technical basis for predicting bioavailability of cationic metals to benthic organisms based on pore water metal concentrations and metal-AVS relationships. Included are discussions of the advantages and limitations of metal bioavailability predictions based on these parameters, relative both to site-specific assessments and the development of sediment quality criteria.     

Using Sediment Budgets to Investigate the Pathogen Flux Through Catchments

We demonstrate a materials budget approach to identify the main source areas and fluxes of pathogens through a landscape by using the flux of fine sediments as a proxy for pathogens. Sediment budgets were created for three subcatchment tributaries of the Googong Reservoir in southeastern New South Wales, Australia. Major inputs, sources, stores, and transport zones were estimated using sediment sampling, dam trap efficiency measures, and radionuclide tracing. Particle size analyses were used to quantify the fine-sediment component of the total sediment flux, from which the pathogen flux was inferred by considering the differences between the mobility and transportation of fine sediments and pathogens. Gullies were identified as important sources of fine sediment, and therefore of pathogens, with the pathogen risk compounded when cattle shelter in them during wet periods. The results also indicate that the degree of landscape modification influences both sediment and pathogen mobilization. Farm dams, swampy meadows and glades along drainage paths lower the flux of fine sediment, and therefore pathogens, in this landscape during low-flow periods. However, high-rainfall and high-flow events are likely to transport most of the fine sediment, and therefore pathogen, flux from the Googong landscape to the reservoir. Materials budgets are a repeatable and comparatively low-cost method for investigating the pathogen flux through a landscape.