Size-dependent effects of dams on river ecosystems and implications for dam removal outcomes

Understanding the relationship between a dam's size and its ecological effects is important for prioritization of river restoration efforts based on dam removal. Although much is known about the effects of large storage dams, this information may not be applicable to small dams, which represent the vast majority of dams being considered for removal. To better understand how dam effects vary with size, we conducted a multidisciplinary study of the downstream effect of dams on a range of ecological characteristics including geomorphology, water chemistry, periphyton, riparian vegetation, benthic macroinvertebrates, and fish. We related dam size variables to the downstream-upstream fractional difference in measured ecological characteristics for 16 dams in the mid-Atlantic region ranging from 0.9 to 57 m high, with hydraulic residence times (HRTs) ranging from 30 min to 1.5 years. For a range of physical attributes, larger dams had larger effects. For example, the water surface width below dams was greater below large dams. By contrast, there was no effect of dam size on sediment grain size, though the fraction of fine-grained bed material was lower below dams independently of dam size. Larger dams tended to reduce water quality more, with decreased downstream dissolved oxygen and increased temperature. Larger dams decreased inorganic nutrients (N, P, Si), but increased particulate nutrients (N, P) in downstream reaches. Aquatic organisms tended to have greater dissimilarity in species composition below larger dams (for fish and periphyton), lower taxonomic diversity (for macroinvertebrates), and greater pollution tolerance (for periphyton and macroinvertebrates). Plants responded differently below large and small dams, with fewer invasive species below large dams, but more below small dams. Overall, these results demonstrate that larger dams have much greater impact on the ecosystem components we measured, and hence their removal has the greatest potential for restoring river ecosystems.

Short-term variations and correlations in water quality after dam removal in the Chishui river basin

Dam damming has an adverse effect on river connectivity, leading to downstream nutrient transport and ecosystem fragmentation. Dam demolition has already been used as an effective measurement to promote the ecological restoration of rivers. Few studies have analyzed the short-term variations of water quality following dam removal. This study investigated the response of multi-element and multi-form water quality parameters, such as water temperature (TEM), dissolved oxygen (DO), pH, biochemical oxygen demand (BOD₅), chemical oxygen demand (COD), ammonia nitrogen (NH₃-N), total nitrogen (TN) and total phosphorus (TP), to the demolition of 4 dams in Chishui River Basin in short term. The study employed Spearman correlation analysis and Generalized Additive Models to identify the critical variables and examine the inter-relationship between these water quality parameters. Our results show that COD, BOD_5, and TP increased after two weeks of dam removal, while NH₃-N and TN decreased. Dams with larger volumes and higher heights led to more obvious deterioration for DO, COD, and BOD₅. We also found that denitrification and resuspension dominantly affect the water quality indicators following dam removal. Denitrification is responsible for downstream TN increase, and resuspension and related sediment transport contribute to downstream TP increase. Our study provides an opportunity to explore the transformation and migration of N and P in reservoirs following dam removal in the short term and presents a scientific basis and new thought for the short-term protection and management following the clean-up and rectification of multiple small hydropower plants.

Consideration of spatial and temporal scales in stream restorations and biotic monitoring to assess restoration outcomes: A literature review, Part 2

Stream and river restoration practices have become common in many parts of the world. To answer the question whether such restoration measures improve freshwater biotic assemblages or functions over time, and if not, can general reasons be identified for such outcomes, we conducted a literature survey and review of studies in which different types of stream restorations were conducted and outcomes assessed. In the first paper, we reviewed studies of culvert restorations, acid mine drainage or industrial pollution restoration; and urban stream restoration projects. Here, we review studies of restoration via dam removal, changes in dam operation or fish passage structures; instream habitat modification; riparian restoration or woody material addition; channel restoration and multiple restoration measures and develop some general conclusions from these reviews. Biomonitoring in different studies detected improvements for some restoration measures; other studies found minimal or no statistically significant increases in biotic assemblage richness, abundances or functions. In some cases, untreated stressors may have influenced the outcomes of the restoration, but in many cases, there were mismatches in the temporal or spatial scale of the restoration measure undertaken and associated monitoring. For example, either biomonitoring to measure restoration effects was conducted over a too short a time period after restoration for effects to be observed, or the sources and stressors needing remediation occurred at a larger catchment scale than the restoration. Also, many restoration measures lack observations from unimpaired reference sites for use in predicting how much of a beneficial effect might be expected.

Associations between riffle development and aquatic biota following lowhead dam removal

Dam removal is an increasingly common river restoration option, yet some of the mechanisms leading to ecological changes remain unquantified. We assessed relationships between riffle structure and benthic macroinvertebrate and fish assemblages 2 years after a lowhead dam removal in Ohio, USA. Hydrogeomorphic, water-chemistry, and biotic surveys were conducted at seven study riffles at six time intervals from spring 2014 through summer 2015. The density and diversity of macroinvertebrates and fish were significantly different over time, largely as a function of season (lowest densities in early spring, greatest in summer). Macroinvertebrate, but not fish, assemblage composition was different by time but not riffle. Although hydrogeomorphic characteristics (e.g., streamflow velocity, substrate size) were linked to shifts in macroinvertebrates and fish, chemical water-quality parameters (e.g., dissolved oxygen, nutrient concentrations) were also implicated as potential biotic drivers. Our results indicate that riffle habitat development can be an important mechanism related to restoring sensitive species and biological diversity following dam removal.

Effects of Dam Removal on Fish Community Interactions and Stability in the Eightmile River System, Connecticut, USA

New multivariate time-series methods have the potential to provide important insights into the effects of ecosystem restoration activities. To this end, we examined the temporal effects of dam removal on fish community interactions using multivariate autoregressive models to understand changes in fish community structure in the Eightmile River System, Connecticut, USA. We sampled fish for 6 years during the growing season; 1 year prior to, 2 years during, and for 3 years after a small dam removal event. The multivariate autoregressive analysis revealed that the site above the dam was the most reactive and least resilient sample site, followed in order by the below-dam and nearby reference site. Even 3 years after the dam removal event, the stream was still in a recovery stage that had failed to approximate the community structure of the reference site. This suggests that the reorganization of fish communities following dam removals, with the goal of ecological restoration, may take decades to centuries for the restored sites to approximate the community structure of nearby undisturbed sites. Results from this study also highlight the utility of multivariate autoregressive modeling for examining temporal interactions among species in response to adaptive management activities both in aquatic systems and elsewhere.

Fish assemblage response to a small dam removal in the Eightmile River system, Connecticut, USA

We examined the effects of the Zemko Dam removal on the Eightmile River system in Salem, Connecticut, USA. The objective of this research was to quantify spatiotemporal variation in fish community composition in response to small dam removal. We sampled fish abundance over a 6-year period (2005-2010) to quantify changes in fish assemblages prior to dam removal, during drawdown, and for three years following dam removal. Fish population dynamics were examined above the dam, below the dam, and at two reference sites by indicator species analysis, mixed models, non-metric multidimensional scaling, and analysis of similarity. We observed significant shifts in fish relative abundance over time in response to dam removal. Changes in fish species composition were variable, and they occurred within 1 year of drawdown. A complete shift from lentic to lotic fishes failed to occur within 3 years after the dam was removed. However, we did observe increases in fluvial and transition (i.e., pool head, pool tail, or run) specialist fishes both upstream and downstream from the former dam site. Our results demonstrate the importance of dam removal for restoring river connectivity for fish movement. While the long-term effects of dam removal remain uncertain, we conclude that dam removals can have positive benefits on fish assemblages by enhancing river connectivity and fluvial habitat availability.