Dam removal has become a national crusade of sorts lately. This crusade has helped to fuel growth in a discipline that has evolved into a billion-dollar-a-year river restoration industry. The danger here is that a better understanding of how natural streams work and of the proper goal of restoration are lagging behind. Better not to have built a dam in the first place than to suffer all the downsides of removal that were succinctly summarized in the editor’s blog on October 12.
Removal makes sense in the case of aging dams that have served their purpose, provide few benefits, and impede anadromous fish runs. On the other hand, each dam removal proposal should be examined on an individual basis—not by prescriptive zeal that assumes all dams are bad. The dangers of the latter approach are well expressed in a recent article in Science (Vol. 319, January 18, 2008) titled “Dreams of Natural Streams.” In it the author, David R. Montgomery, a well-regarded fluvial geologist, warns that in many cases dam removal has been advocated without any real sense of what constitutes a river’s “natural” condition and how removal will help achieve this restoration goal. He notes, “The classic sinuous form of meandering channels has come to represent a natural ideal in restoration design—even for rivers for which such an ideal is historical fiction.” The author also emphasizes that the first step in a river restoration program should be to develop a solid understanding of what the targeted river was actually like before restoration begins. Instead, the implicit mantra of many restoration projects—namely “enough studying, let’s just fix it”—has been based, according to Montgomery, on the idea of reengineering an archetypal meandering channel form.
These crosscurrents and concerns are very much in play in a proposed dam removal project in Ann Arbor, Michigan, that has split the local community into opposing factions. Argo Dam was built across the Huron River over a century ago. The dam has created a large pond that provides multiple and important recreational benefits to the citizens of Ann Arbor. “Dam Out” proponents tend to assert the benefits of removal without properly considering the costs and downsides of removal that were identified in the editorial blog.
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| Figure 1. Overhead view of existing channel below dam. The old river channel parallels the Amtrak railway and also underlies the MichCon staging area shown in lower right corner of photo. |
In the case of Argo Dam, the most serious overlooked consequence (and potential cost) of removal is the impact on river flow direction. Instead of continuing in a natural southerly direction, the Huron River today turns sharply northward below Argo Dam as shown in Figure 1. This abrupt “fishhook” bend was not always present. Historical maps (dated 1895 and 1943) show that the river channel at one time roughly paralleled the Amtrak railway as shown in Figure 2. This change in direction was confirmed by superimposition of dated geo-referenced maps. The original channel passed under the present location of the MichCon service center (Figure 1). Argo Dam dissipates the energy of the river in a stilling basin at the toe. This dissipation of energy and momentum has allowed the diversion of the channel northward as shown in Figure 2.
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| Figure 2. Huron River channel below Argo Dam. Position of “old” channel bed in relation to river channel and millrace canal in 1943. Sometime between 1895 and 1943 the river was deflected (northward) out of its old channel into its current channel. |
If the dam is removed, the river will attempt to reoccupy its old channel (Figures 1 and 2). Views of the entrance to the original channel, including a thin sheet-pile wall constructed across the entrance, are shown in Figures 3 and 4. Absent Argo Dam, this wall would either be overtopped, flanked, and/or toppled during a high-water flood event. This would result in flooding and inundation of the MichCon service center (Figure 1) and surrounding areas. A coal gasification plant was formerly located here. This site contains buried toxic chemicals and is one of the most hazardous sites in Ann Arbor. The Huron River cannot be allowed to occupy its old channel and flow through this site!
If Argo Dam is removed, river training structures—e.g., bendway weirs—and bank protection measures would be required to confine the river to its present channel and prevent this from happening. Hydraulic modeling studies such as HEC-RAs are required to assess post-dam removal flow conditions and to select necessary training measures. These studies have not been undertaken to date nor construction costs determined. These investigations should precede any serious proposal to remove not only Argo Dam but also any major dam on a river.
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Figure 3. View from Argo Dam of entrance location of original Huron River channel (middle center of photo)
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| Figure 4. Closeup overhead view of Huron River below Argo Dam. Thin sheet-pile wall extending part way across the old river channel is shown at bottom center of photo. |
