Science and dam removal: filling the knowledge gaps10 September 1999
Does dam removal really improve the overall health of a river? Emily H Stanley* reports on a research project which aims to gather evidenceto answer this question
In Wisconsin, US, dam building began in earnest in the early 1900s, and was followed by another boom in the early 1950s. Many communities gained critical economic footholds from mills and small industries associated with hydro power generation. However, many believe that the same structures which initially fuelled the growth of communities have now outlived their usefulness — businesses that once relied on hydro power have, for the most part, come and gone on the American economic landscape. Many dams are now in excess of 50 or even 100 years old and are being re-evaluated by state and federal authorities. Increasingly, natural resource managers are recommending dam removal as a viable management strategy. While this is often unpopular with local communities, the fact is the number of dams being removed in Wisconsin and elsewhere in the US is on the rise.
Despite the widespread assumption by environmentalists that dam removal will improve the overall health of a river, evidence to support or refute this belief is surprisingly scarce. As the State of Wisconsin is faced with an ageing population of dams, and owners are increasingly faced with the decision to repair or remove dams, basic information on removal is desperately needed.
Such a knowledge gap has motivated a group of researchers from the Wisconsin Department of Natural Resources and the University of Wisconsin to band together and investigate ecological conditions before and after the removal of a series of dams on the Baraboo river. Funded by a combination of state sources and the Sand County Foundation (a group that promotes land conservation and stewardship) our research group hopes to understand how fish, invertebrates, water quality, channel form and sediment storage change over a six-year period in which the Baraboo river experiences the removal of four low-head dams. An intensive ‘before and after’ monitoring programme should reveal patterns and rates of change in riverine communities and habitat following this action.
The seemingly straightforward task of predicting changes after a dam is removed is far from simple. Many questions remain unanswered but basic generalisations can be made:
•The impounded area behind the dam will be replaced by free-flowing water, and aquatic communities in the area will change accordingly. But at what rate will fish and invertebrate communities make this shift?
•Will fish such as sturgeon that have been isolated from the river for decades become re-established in newly available habitat?
•How rapidly and how far do fish migrate following removal of the dam?
•How long will it take the large volume of sediments that have accumulated behind a dam over the course of several decades to distribute themselves downstream?
•And what are the effects of this slug of sediments (and any associated nutrients or pollutants) on the downstream ecosystem?
•What of wetlands that have become established around the margins of the reservoir?
•How does channel form and habitat quality change over time?
•How much of the river is affected by a single dam removal?
These questions will be addressed by our group of fisheries and aquatic scientists in this in-depth study. The fish group — headed by researchers Tom Pellett from Wisconsin Department of Natural Resources (DNR), and Mike Bozek from the University of Wisconsin at Stevens Point — will use a two-pronged approach to gathering data:
•First of all, fish communities will be assessed through standard practices (eg collecting fish along stretches of rivers using mini-boom shockers) throughout the entire length of the river before, during and after dam removal. This will allow changes in community structure over time to be evaluated.
•Secondly, there will be a massive campaign to tag fish throughout the lower reaches of the river and to study fish migration patterns. The hope is that changes in fish community structure can be understood by knowing where fish came from or went to along the length of the river.
Macroinvertebrate samples are being collected using both qualitative (kick nets) and quantitative techniques (sampler and grab). For this work David Marshall of DNR is using monitoring approaches that have been developed for statewide use. At this point, the intent is to be able to identify major shifts in macroinvertebrate communities. For example, following the removal of the first dam on other rivers, Marshall has seen worm and midge larvae communities decline, and mayflies and caddisflies increase.
Water quality is being monitored by monthly sampling at four points along the river, including sites that are above and below the set of dams in the City of Baraboo. Also, a series of two-week profiles within each of the impoundments are being collected for oxygen, pH, temperature and conductivity using a set of automated, continuously recording water quality probes.
Channel form throughout the lower section of the river is being documented by establishing a series of fixed cross-sections that will be re-surveyed following each dam removal. Sediment composition along these transects will be quantified as well. Movement of sediments will be monitored during the actual breaching, at regular intervals thereafter, and during floods (when most sediment will be moved) using bedload and suspended load sampling bottles.
Completion and publication
At the moment, the research group has a six-year outlook for the project; this will cover all dam removals plus some recovery time. Any results will be published as work progresses. For example, work on sediment transport and channel adjustment will probably be submitted for publication within the next two to three years, and some of the initial descriptive (pre-removal) work is also likely to be written up within the next one or two years.
Many of us would like to believe that with old and unproductive dams, removal represents an across-the-board improvement in the river. However, as is the case with virtually all ecological restoration efforts, trade-offs are expected. An action that results in one benefit (such as an improved fishery) may come with a cost such as loss of recreational values provided by the reservoir. Understanding and accepting these trade-offs is an essential part of understanding the entire dam removal process, yet this human context of dam removal is virtually unstudied.
Wisconsin is not alone in facing the re-evaluation of dams that are now 50 years old or older, nor in its decision to remove some of these structures. Yet in many cases, authorities may delay the final decision of repair or removal until more is known about each management option. To date, most studies of dam removal exist in the ‘grey literature’; that is, in reports and government documents that do not receive wide circulation and are difficult to obtain. The Baraboo study will provide one of the first ever in-depth, long-term examinations of dam removal, and is unusual in its consideration of a broad suite of environmental attributes.
Although still in its first year, the unique features of the research project and the growing interest in dam removal has generated wide interest in the Baraboo river study. Consulting firms, researchers, state resource management agencies and hydro power industry employees across the US have expressed interest in the project. Consultants from New Zealand, who may be evaluating ageing dams sometime in the near future, have also expressed their interest.
As we learn more about both the short- and long-term changes that occur when a dam is breached, it will be essential to disseminate the information widely through management reports, scientific literature and presentations at water resource management meetings. And while the Baraboo River Research Project will provide a critical insight into dam removal, this study must only be considered as an initial step toward a broad understanding of removals — more studies are needed across a range of geographic locations, rivers and dam sizes.
Dams and their impoundments will always play a fundamental role for us, but their lifespan mandates that such structures will come and go over time. Basic research is badly needed to understand the changes that occur when dam breaching is upon us.