Estabrook Dam’s Environmental Impact
Green activists support the dam’s removal, but that might actually hurt the environment.
Dams can certainly have negative environmental impacts.
They can serve as barriers to the migration of certain kinds of fish (those that undertake regular migrations in large freshwater systems). They can function as traps for the accumulation of sediment and pollutants. They can negatively impact water quality by raising temperatures and lowering dissolved oxygen levels.
Moreover, they have often outlived their purpose. In Wisconsin, many dams still in use were built in the 1800s to provide a reliable and controlled source of water to power various types of mills or early industrial facilities. In terms of their original purpose, these dams are now obsolete. Such was the case with the North Avenue Dam, which was removed from the Milwaukee River in 1994 and resulted in widely reported improvements in the ecological health of the former impoundment, as best demonstrated by an increase in the number of fish species from eight to more than forty.
When it comes to the Estabrook Dam, an assumption shared by all parties is that its repair — should that option be chosen — would occur in conjunction with construction of a fish passage, addressing one of the key environmental concerns associated with the dam. But even with construction of a fish passage, the dam could in theory cause an array of other negative environmental impacts, including: (a) buildup of sediment behind the dam and within the impoundment; (b) potential impacts of the seasonal drawdown on mussels and other aquatic life in the impoundment (through desiccation, predation, and freezing of the mussels); and (c) general degradation of water quality and wildlife habitat within the impoundment. The environmental assessment (EA) prepared for the dam expands on this last point: “Specifically, the replacement of a naturally flowing stream channel with an essentially stagnant pool of water eliminates extensive wetlands and floodplains by covering them with water. It also results in higher water temperatures and lower dissolved oxygen levels. Other impacts include sedimentation, degraded sediment, and impacts to stream channel structure. These conditions have been identified as a major cause for the decline in fish community diversity, abundance, and structure in Wisconsin. These cumulative impacts can directly and indirectly impact the overall viability of individual fish species, reproduction, food supply and growth, predation, resistance to disease and stress, etc.”
The Estabrook Dam would seem to be an ideal situation for demonstrating the benefits of dam removal, as the free-flowing conditions that would result have already existed for more than seven years as a result of the WDNR order to open the gates in 2008. Unfortunately, almost no water quality or ecological data appears to have been collected from the impoundment area during this time that would document increases in oxygen levels, fish and mussel populations and diversity, or decreases in peak summer water temperatures. As a consequence, most of the environmental arguments in support of removal are based on general assumptions rather than analysis or data specific to the Estabrook Dam. And these assumptions are worth examining in detail.
Sediment Accumulation Concerns
One of the most significant environmental concerns if the dam is repaired is the potential for long-term accumulation of sediment within the impoundment as well as the seasonal release of sediment that would occur when the gates are opened each fall. The amount of sediment transported by a river is dependent in part on the flow velocity of the water. When a river enters an impoundment formed by a dam, the velocity of the water decreases, resulting in an increase in the amount of sediment settling from the water. Evidence of this process is visible within the former impoundment area of the North Avenue Dam, where several hundred thousand cubic yards of sediment were left in place following dam removal. Typically, the water level in a dam’s impoundment is lowered each fall to reduce the risk associated with accumulation of ice behind the dam during winter months. Even if the gates for the dam are gradually opened, the flow of water within the impoundment area would increase for a period of several days, potentially resulting in erosion of some of the accumulated sediment and transport of this sediment downstream where it could harm habitat areas.
It should be noted that the accumulation of sediment along a river is not always a concern, as the transport of sediment is a dynamic process occurring at nearly all times in every river, with erosion typically occurring in areas of the river where current is flowing the fastest, and the accumulation of sediment occurring where flow is relatively slower. The key issue for Estabrook Dam is whether the repair of the dam, and the continued existence of impoundment, would result in significant net accumulation of sediment over time. This question does not appear to have been addressed in any of the evaluations performed to date.
Reviewing some of the scientific literature on dams, it turns out it is possible to estimate the “efficiency” of an individual dam impoundment in trapping sediment based on the ratio of the normal reservoir capacity to the mean annual flow volume. According to Dam Removal Research: Study and Prospects, “Reservoirs with a capacity to store more than 10 percent of the average annual inflow would be expected to trap between 75 and 100 percent of the inflowing sediment. Reservoirs with a capacity to store 1 percent of the average annual inflow would be expected to trap between 30 and 55 percent of the inflowing sediment. When the reservoir storage capacity is less than 0.1 percent of the average annual inflow, the sediment trap efficiency would be nearly zero.” The greater the relative size of the reservoir, the greater its sediment trap efficiency and the amount of reservoir sedimentation that will occur. For example, the Hoover Dam forms an impoundment (Lake Mead) that is up to 590 feet deep and which took nearly six years to fill following completion of the dam. Water within Lake Mead therefore has an approximate six-year residence time, resulting in conditions in which essentially 100 percent of sediment is captured by the reservoir.
So the dam may present no problem of sediment accumulation. And historical records as well as recent observations suggest that may be the case. A deep swimmable pool reportedly existed in this section of the Milwaukee River prior to construction of the dam, even though the river had been impounded for hundreds if not thousands of years by the natural dam formed by the rock ledge that was removed in the 1930s as part of the channel deepening project performed in conjunction with dam construction. The lower portion of the impoundment was largely clear of sediment when surveyed in 2010 as part of planning for the second phase of contaminated sediment removal project, with only 6-inches of sediment present above the bedrock beneath the I-43 bridge. In the portion of the river lying south of Hampton Avenue, the main channel “contained a very thin to nonexistent sand layer of sediment above a bedrock subsurface.” All this suggests the flow dynamics within both the natural and man-made impoundment are sufficient to keep significant portions of the impoundment largely free of sediment.
In fact, its possible that removal of the dam might actually exacerbate accumulation of sediment within the former impoundment area. Nearly 30 acres of the former impoundment will eventually become covered by trees and other vegetation if the dam is permanently removed. This process may already be occurring as a consequence of the drawdown in place since October 2008. Significant accretion of sediment in vegetated areas could potentially occur over the next one or two decades before an equilibrium condition is established between the growing (and eventually mature vegetation), flood flows, and the channel geometry. The Southeastern Wisconsin Regional Planning Commission reportedly adjusted their model for changes in “roughness” associated with the newly vegetated areas but not for changes in channel geometry that could occur as a result of sediment accretion.
In conclusion, there may be no benefit in removing the dam in terms of sedimentation issues, and there is at least the possibility that removal of the dam could exacerbate sedimentation over the next decade as the channel in this area responds to the continued maturation of vegetation on approximately 30 acres of former impoundment.
The Occurrence of Stagnant Conditions within the Impoundment
A general environmental concern if the dam is repaired is that water within the impoundment will be stagnant. To better understand the behavior of water within the Estabrook Dam impoundment, and the relative degree of stagnation, I calculated the residence time for water in the impoundment for different flow rates that have occurred historically, as well as projected flows when flooding occurs.
|Estimated Residence Time for Water in the Estabrook Dam Impoundment for Different Flow Rates|
|Flow Rates||Approximate Residence Time||Comments on Flow Rates|
|Cubic Feet per Second||Cubic Feet per Hour||Cubic Feet per Day||Days||Hrs.|
|117||421,200||10,108,800||3.0||72.5||Mean minimum summer flow (2008-14)*|
|240||864,000||20,736,000||1.5||35.4||Median flow (1915-2014)**|
|451||1,623,600||38,966,400||0.78||18.8||Mean flow (1915-2014)**|
|619||2,228,400||53,481,600||0.57||13.7||Mean summer flow (2008-14)*|
|4,730||17,028,000||408,672,000||0.075||1.8||Flow rate for 2-year flood|
|8,790||31,644,000||759,456,000||0.040||1.0||Flow rate for 10-year flood|
|14,800||53,280,000||1,278,720,000||0.024||0.6||Flow rate for 100-year flood|
|18,200||65,520,000||1,572,480,000||0.019||0.5||Peak recorded flow (7/22/2010)|
|* Calculated by author using reported flow data for the Estabrook Park flood gage (U.S.G.S. Gage 04087000for the period May 15 to September 15 of each year.** Data as reported in the Draft Environmental Assessment Report dated August 2014 prepared by AECOM. Residence times are calculated using the reported volume for the impoundment (with gates closed) of 700 acre-feet (=30,555,000 cubic feet).|
The table is less accurate for higher flows occurring during major flood events when the volume and reach of the river would be significantly increased. Still, the numbers are useful for illustrating that at the long-term mean flow rate of 451 cubic feet per second (cfs), water in the impoundment would have a residence time of less than one day. During summer months when the gates would be closed, based on the mean summer flow from May 15 to September 15, the flow of water in the impoundment would still be great enough to result in the replacement of the full volume of water approximately two times each day. Approximately 290 million gallons of water per day move through the impoundment at the mean flow rate of 451 cfs, and over 400 million gallons per day at the flow rate of 619 cfs. Therefore, even with the dam in place, water continues to flow through this section of the Milwaukee River at a slow but significant rate.
The flow data are also relevant to concerns associated with seasonal lowering of the water level in the impoundment. A 10 percent increase in the mean flow rate of 619 cfs would be sufficient to reduce the volume of water in the impoundment by 50 percent over a six-day period. The magnitude of the increase in flow necessary to partially drain the impoundment is dwarfed by the magnitude of flow changes that occur each year associated with even minor flood events (for example a 2-year flood event with a flow of 4,730 cfs) and which would continue to occur even if the dam is removed.
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