St Croix Dam

OVERVIEW	HISTORY	ECOLOGICAL PROBLEMS	FIXING IT	FURTHER READING

OVERVIEW

The St. Croix River is one of the most diverse riverine ecosystems in the Upper-Midwest. It is home to 68 fish species, 39 mussel species, and 497 macroinvertebrate taxa (Fago and Hatch 1993). Unfortunately, many of these species have been classified as threatened or endangered on both a federal and state level (a complete list of the endangered species). There are a total of thirty-nine at risk species including the crystal darter, the St. Croix snaketail dragonfly, and the winged mapleleaf mussel (see below).

One of the major threats to all of these species is the St. Croix Falls hydroelectric project. The dam haseight turbines each with a hydraulic capacity of 800 cfs and it generates 119.6 million kilowatt-hours of energy annually. It is operated by Xcel Energy as a peaking facility which means that in the winter the flow is regulated based on power demand. During periods of low demand, the company stifles the flow and when demand increases they release the water that has collected. This peaking behavior along with other ecological hazards associated with the dam have detrimental effects on many of the taxa residing within the river system. Over the past 3 years there has been an increased interest in mitigating some of the problems associated with the dam, but there has been no outright success.

This map shows the upper and lower St. Croix River with the St. Croix Falls Dam.

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HISTORY

The section of the river that is impounded is located just north of Hudson, Wisconsin. This area was originally blocked by the Nevers Dam in 1890 as a means of controlling the log jams that plagued the river. By 1902 the timber had been depleted and so the dam was to be abandoned, but Northern States Power stepped in and bought it in order to control flow while they constructed the hydroelectric plant at St. Croix Falls, just down river. The plant was completed in 1907 but the Nevers Dam was not removed until 1954 when it was deemed unsafe and a nuisance to recreational activities. Since it was built before the Federal Power Act of 1920 the Federal Energy Regulatory Commission does not oversee its use, instead it falls under the Federal War Powers Act of 1903. Under this Act, Xcel is required to maintain a flow of 1600 cfs from April 1 through October 31. Since 1989 Xcel has voluntarily released at least 800 cfs during the remainder of the year.

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ECOLOGICAL PROBLEMS

The literature documenting the ecological damage caused by aquatic impoundments is vast; there are a number of problems that dams can cause to an aquatic ecosystem. First, they interfere with migration patterns, particularly fishes’ spawning patterns by preventing movement upstream. Next, they can create excessive mixing below the blockage, leading to a saturation of oxygen and may cause blood-oxygen disease. The disturbances they create in the tailwaters can change the substrate as well as the thermal environment. Above the blockage, there is extreme sediment deposition and significant changes in the nutrient content. These changes can be classified as both habitat alterations and sporadic disturbances, both of which lead to shifts in biodiversity and overall population concentrations around the impoundment.

Given that the St. Croix Falls dam is operated as a peaking facility the taxa are at particular risk from rapid flow changes. The minimum discharge of 800 cfs during the winter is not enough to cover the beds and so the river is subject to artificial drought conditions. The only occasion since 1902 when 800 cfs occurred naturally was August of 1934 so the flora and fauna are not well adapted to deal with this problem. Droughts in the winter months expose the substrate and create a dewatered zone. During the winter of 1989 the beds below the dam were completely exposed at night and in 1991 the Wisconsin DNR found a layer of ice 13 inches thick directly on the gravel in the bed (Eldridge, 1991). The literature has shown that such dewatering can be detrimental to fish, insect and mussel diversity. Blinn et al (1994) showed that a substrate exposed to air for 2-12 hours requires 4 months to regain biomass similar to the unexposed habitat.

The Minesota and Wisconsin DNR conducted a dual-flow analysis to better understand the effect of peaking on biodiversity. They attempt to bracket the peaking impact based on the organism’s ability to move in response to rapidly changing flow conditions (Benike et al., 2000). The study found that effective habitat for all species increased as baseflow increased, which implies that peaking could be very harmful to biodiversity in the region.
dragonfly larva

The threat to larval insects is dependent upon how the timing of the peaking lines up with the timing of emerging behavior. If the insects hatch during periods of high flow, they will not be harmed when the flow is reduced. However, if they hatch during the off-peak periods they can be washed away when the water is released for generation. These impacts are more important to species with synchronous hatching strategies, such as dragonflies, since the flow regime will then effect entire populations. There are several species of endangered dragonflies living around St. Croix Falls and so the peaking could be very harmful to the system as a whole.

Of particular concern to conservationists are the winged maple leaf mussels, which reside in the tailwaters beneath the dam. This is the last known population of this species of mussels and so their preservation is considered a very high priority. Historically, they were found in large to medium sized clear-water streams but most of these have been lost to channelization. The mapleleafs are also threatened by agriculture and modified land use (Eldridge, 1991). Hornbach et al. (1996) showed that winged mapleleaf mussels require roughly the same habitat as other mussel populations. The main difference is that they prefer a slower bottom current, which could make them even more susceptible to population loss due to peaking. Since they follow the general mussel population it is helpful to understand how impoundment affects overall mussel habitats. mussels

The construction of dams is inherently bad for mussel populations; benthic diversity generally declines both above and below impoundments (Yeager, 1993) and several dozen mussel species have been driven to extinction wholly or in large part through the construction of dams (Watters, 1999). Mussels need a very specific depth, temperature, and substrate to survive and reproduce and dams modify all three of those characteristics. There are two areas around a dam where habitat modification can occur: Above the dam and below the dam in the tailwaters. The tailwaters are obviously most directly affected by peaking, but the area above the dam also undergoes changes after impoundment.

Above the dam the original channel usually remains intact but with much deeper waters and significant changes to the substrate. To begin, mussels cannot survive in water that is too deep, so some will die immediately. Since the water slows just before the dam, sediment tends to build up and can smother mussel populations that cannot adapt to the softer substrates. Ellis (1936) found that silt deposits above dams result in loss of light penetration and ultimately reduce algal abundance. Since periphytes are an important food source for mussels these changes can result in starvation and population loss. He also found that the hypolimnion gets cooler since the water is deeper and there is less light. Mussels’ reproduction is temperature dependent and if the water is too cold they will not reproduce, which is another source of population loss.

Below the dam the mussels are exposed to the dual-flow system. Peaking is most harmful to species with limited capacity to move in response to receding water levels. The rapid changes in flow that come with peaking can expose mussels to extreme temperatures, which can be fatal. Additionally, the changes in velocity can harm mussels that are not adapted to handle fast currents, such as the mapleleaf. In DNR surveys, divers observed no mussels in the area of the lake affected by the rapid drawdowns; the few mussels that were that were found were fast-growing and short-lived species (Benike et al., 2000).

One potentially advantageous effect of impoundments on river ecosystem is that they act as barriers to mussel populations. They impede fish movement and since fish are important to mussel reproduction (glochidial parasites) certain species of mussels cannot get hosts upstream from dams. This effect could help stop the invasion of zebra mussels from spreading too wildly in the St Croix.

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FIXING IT

In order to help mitigate the damages to endangered species living near the St. Croix Falls dam, Xcel Energy signed a memorandum of understanding (MOU) in which they agree to switch from peaking to run-of-river management. Under the new management system, the flow into the dam should always equal the flow out of the dam. While there would be no net power loss, Xcel Energy would stand to sustain some economic losses given that they would be generating less high-demand peak power. The Minnesota DNR found that the dam must release 1,980 cfs to adequately protect the riverbed from dewatering. Presumably, under run-of river management this level would be sustained.

There are some that believe that such a switch would not be beneficial to the winged mapleaf mussels. They argued that the reason for the winged mapleleaf’s success beneath the dam is that they are not exposed to toxicants being held back by the impoundment. This minority makes an argument consistent with the Precautionary Principle - that a conversion to run-of-river management will release the pollutants too rapidly, in turn, harming mussel population further. There has been no evidence to support this claim and most believe that a rapid transition is acceptable.

The biggest problem with the MOU is that it is impossible to enforce its guidelines. Xcel claims that they are not governed by the authority granted to the DNR in Wisconsin Statute Chatpers 30 and 31, which normally grants the DNR authority to regulate and control the level and flow of water in all navigable waters. Since neither the National Wild and Scenic Rivers Act nor the Endangered species act can grant authority to an agency to regulate flows, the power company has been able to avoid legal scrutiny. Even though the MOU is nonbinding, the hope is that Xcel will follow its terms and conditions because if they fail to meet its flow regimes the DNR could issue a binding flow order. Indeed, the there is evidence that Xcel has failed to meet the conditions since the flow has fallen below what is regarded as normal for the river and the DNR has yet to issue any binding resolutions (see graph).

A graph of the dam's discharge. It should not fall below ~2,000 cfs after the 2006 MOU.