Case Studies: Army Corps of Engineers Dam Operations

In 2002, The Nature Conservancy joined the Army Corps of Engineers in a collaborative national program called the Sustainable Rivers Project. We are working with the Corps to assess opportunities for modifying operations of 16 dams on 12 rivers to improve ecosystem health.

Savannah River, Georgia and South Carolina
During the past 18 months, we have been working with scientists at the University of Georgia (UGA) and 11 other agencies and organizations to develop ecosystem flow recommendations for the Lower Savannah River, which forms the border between Georgia and South Carolina. This project, supported by the U.S. Army Corps of Engineers and the two state governments, has engaged more than 40 scientists to date in a collaborative effort to define the flows needed to support freshwater, floodplain, and estuary ecosystems. During a workshop in early April 2003, scientists developed a flow prescription that addresses low flows, high flow pulses, and flood flows in different reaches of the river. The scientists also identified critical research and data collection that will help create a foundation for a longer term adaptive process for refining flow recommendations. Download a paper (PDF) summarizing the Conservancy's work on the Savannah River.

Green River, Kentucky
Winding its way through the interior plateaus of south-central Kentucky, the Green River watershed is rich in aquatic biodiversity, supporting 151 freshwater fishes (12 of which occur only here), 59 mussels, and an unparalleled collection of cave flora and fauna. The predominant threat to the river is hydrologic alteration, and the primary source is the Green River Dam in the river’s headwaters. For more than thirty years the Army Corps of Engineers strictly regulated lake levels, downstream flows, and water temperatures for recreation, irrigation, and flood control. Now The Nature Conservancy is working with the Army Corps of Engineers to alter their operation of the Green River Dam to restore natural flow regimes to benefit wildlife. Read the full story below to find out more about the Green River and how this partnership came about.

A postcard printed in 1910 pictures the steamboat Chaperon docked at a river wharf, carrying hundreds of passengers up the Green River in Kentucky to visit one of the world's great wonders - Mammoth Cave. Women in long white dresses and men sporting straw boater hats hang from the boat's railings and bustle about on the wharf, excitedly anticipating their excursion to the longest cave on the planet. Earlier visitors had enticed them with tales of the "appalling spectacle" of the cave's "nether world," from which nitrous odors and eerie puffs of wind would emanate.

But like most people of their day, these tourists were unaware that the navigation locks and dams which made possible their steamboat's passage up the shallow river were destroying one of the world's treasure chests of aquatic biodiversity. As has been the case with so many of the world's rivers, ecological crises occurring beneath the water's surface have escaped attention for long periods of time - in this case, for nearly a century. Fortunately, the Green River's story illustrates the growing role of science in heightening public awareness of what is being lost from river systems. It also demonstrates how the technical skills and ingenuity of water engineers can be harnessed in restoring them.

In 1906, just a few years before the photo of the steamboat Chaperon was taken, the U.S. Army Corps of Engineers had built a structure called Green River Lock and Dam 6, at a point nearly 300 kilometers upriver from the Green River's confluence with the Ohio River and just downstream of Mammoth Cave. (Figure 4-8.) Steamboats had been operating on the Ohio since 1811, but only with Lock and Dam 6 in place were the big boats able to reliably make their way over the mussel-laden shoals of the Green to reach the cave. The new lock and dam backed up the river's flow and raised water levels in Mammoth Cave by nearly four meters, jeopardizing one of the world's richest and rarest assemblages of cave animals, as well as many more species in the Green River itself.

Mammoth Cave's origins date back 350 million years, to a time when the continents were all joined in one huge landmass called Pangaea. The warm waters of the Panthalassa Ocean teemed with corals, shellfish, snails and other tiny organisms whose shells were made of calcium carbonate. As these creatures died, their shells accumulated on the floor of the ancient sea. Over 70 million years, the build-up of shells and muck created enough pressure to weld the mixture into limestone rock. Then, 280 million years ago, the sea level dropped and exposed the limestone, setting the stage for the formation of Mammoth Cave. Movements in the earth's crust caused the limestone to rise, buckle, and twist, forming cracks in the rock. Rainwater began seeping into the cracks, dissolving the limestone and forming tunnels, underground rivers, and huge caverns.

A rich variety of animals colonized the cave's unusual environs, in which the air temperature and humidity differed greatly from the outside world. More than 200 animal species now occupy the cave. Some of the most extraordinary creatures include 42 species of troglobites, animals that are adapted exclusively to life in the darkness and cannot live outside of caves. With no light in the cave's depths, many fish and crayfish evolved into eyeless beings with other highly developed sensory organs. With no need for camouflage or protection from the sun, many lost their skin pigment and turned white.

The Green River itself harbors one of the United States' most diverse assemblages of fish and freshwater mussels. More than 150 fish species and more than 70 mussel species have been found in the river. Many of their names sound fanciful, like fictional creatures out of a Dr. Seuss book - fish called the mooneye and the northern hog sucker, and mussels like the purple wartyback, elephant ear, sheepnose, and monkeyface. One does not have to use much imagination to know how the white heelsplitter got its name.

Today, more than one-third of the fish species in the Green River are considered rare, threatened, or endangered at the state or federal level. Fourteen mussel species have disappeared from the river in recent decades. The Army Corps of Engineers itself attributes the loss of these mussel species to the lock and dam structures it owns and manages on the river. The agency built three more of these structures after it completed Lock and Dam 6. However, Lock and Dam 4, located 250 kilometers upstream from the river mouth, collapsed in 1965 and all navigation above Lock and Dam 3 was discontinued in 1981. In 2001, the Corps proposed removing Lock and Dam 6 to restore ecological conditions in the Green River and Mammoth Cave. In its environmental assessment of the dam removal proposal, the Corps explained that the locks and dams had caused a shift in river habitats from cool, free-flowing conditions to slower-flowing, warm-water conditions, to the great detriment of many of the river's species. Riffle and shoal areas with sand and gravel beds were eliminated by the navigation dams, replacing them with silty-bottomed artificial pools.

By removing Lock and Dam 6, the Corps can restore 27 kilometers of river habitat. In addition, the endangered Kentucky cave shrimp and innumerable other creatures in Mammoth Cave will likely benefit from the restoration of their habitats.

Removing Lock and Dam 6 is a critically important step in the ecological restoration of the Green River and Mammoth Cave. But in 2000, at about the same time the Corps was preparing its proposal to remove Lock and Dam 6, The Nature Conservancy conducted an analysis of its own. Its study identified a much bigger problem with the river's hydrologic system.

Two hundred kilometers upstream of Lock and Dam 6 sits the Army Corps' Green River Dam, built in 1969 to provide flood control and recreational benefits. Along with 47 other dams, the Green River Dam was authorized by the national Flood Control Act of 1938, but was only funded many years later, after a devastating flood hit the region in 1962. Unlike the small navigation dams downstream, which back up water during low-flow periods but have little impact on floods, the 43-meter high Green River Dam completely plugs the river, stopping every drop of water moving downstream. The Corps' dam operators then release water from the dam through a concrete pipe 5 meters in diameter. Since 1969, the Green River's flow has been completely under the control of the Corps.

The Nature Conservancy's analysis in 2000 of the dam's impacts on Green River flows revealed that the Corps was releasing water from the reservoir in a way that resembled natural flows during most of the year. In most months, the Corps simply released water at the same rate at which it entered the reservoir. But during the spring and autumn, flows were being altered substantially (Figure 4-9.). These changes in flow resulted from trying to manage the reservoir for both flood control and recreational benefits. During summer, the reservoir was maintained at a high level to maximize the surface area of the lake for fishing and recreational boating. But during the winter, the lake was lowered by more than three meters to create space in the reservoir to store winter floodwaters.

The transitions between these two reservoir levels were accomplished during the spring and autumn. In spring, when trying to raise the level of the reservoir to get ready for summer recreational use, much of the flow of the Green River was captured in the reservoir, thereby reducing flows downstream. Conversely, a considerable volume of water needed to be released from the reservoir during the autumn in order to prepare to capture winter floods. The spring transition took four weeks, and the one in autumn took ten, with most of the autumn release occurring between October 16 and December 1. During those sharp transitions, the river took a beating.

The Conservancy scientists believed that the flow alterations associated with the dam were causing serious problems for the Green River's mussels and fish. In particular, they were concerned about the autumn transitions. During a time when the river's flow would naturally have been low, the large dam releases in autumn created a sustained high flow in the river, nearly filling its channel. Figure 4-9 shows that in some years the dam-released October flows were five times higher than any of the pre-dam flows in that month. The Conservancy scientists were concerned that these prolonged, out-of-season high flows were harmful to fish spawning and mussel reproduction occurring during this time, perhaps even flushing some mussels and small fish downstream. They decided to discuss these issues with the Corps. A meeting with the Corps' Louisville District office was set for June 8, 2000.

On the day of the meeting, the Conservancy scientists met early to discuss their game plan. The group's hydrologist counseled the other scientists not to expect too much from the first meeting with the Corps. Their goal would be to gain a better understanding of the Corps' objectives, not to push for desired changes immediately. The scientists expected the Corps' engineers to describe their operating constraints - why the reservoir needed to be maintained at a certain level for recreation, why they needed ample flood control storage, and so forth. The scientists expected the engineers to be unyielding in maintaining the status quo. They were in for a big surprise.

After the scientists opened the meeting with some basic questions about the dam's operations, Bob Biel, the Corps' senior dam engineer in the Louisville District, asked a pointed question: What exactly did the scientists think the Corps was doing wrong? The scientists floundered for a moment, not anticipating such a direct query. Then they carefully began explaining how the river's plants and animals depend upon natural river flow conditions, and why the dam-altered flow patterns were problematic. They brought out their graphs depicting changes in river flows since the dam's construction, and suggested that they would like the Corps to modify their dam operations to alleviate some of the impacts.

Biel responded by explaining what the dam was built to do - control floods and provide for recreational uses. He further explained that marina owners had constructed their facilities in accordance with the expected summer reservoir level, and lowering that level would be very costly and contentious. Then, just as the scientists began to feel their pessimism resurface, Biel jumped out of his chair, went to the chalkboard, and began drawing a picture of how both organizations could get what they wanted.

First, Biel suggested that the winter reservoir level could be raised by a modest amount - about a meter or a little more. By reducing the difference between summer and winter reservoir levels, they would not need to capture so much water in the spring, nor release so much water in the autumn. Then, if they prolonged the transitions between high and low reservoir levels, they could further minimize the impacts. The scientists were elated. The engineers at the Corps expressed considerable interest in seeing whether they could make their dam operations more ecologically compatible, and were thankful that the scientists had taken the time to explain what they wanted. As the meeting came to an end, Biel said he wanted the scientists to use their expertise to design a monitoring program that could tell the Corps whether modifying their dam operations was making a difference for the river.

Bob Biel retired from the Corps six months after that meeting, ending a 32-year career with the agency. He immediately went to work for the Conservancy under a contractual agreement to provide further assistance on modifying dam operations on the Green River, as well as helping the Conservancy to develop a broader relationship with the Corps. The Louisville District prepared various reports necessary to gain approval of the new plan within their agency and conducted public meetings to get input from landowners along the river. The Conservancy scientists collected extensive ecological data on the health of the river, providing them with an excellent baseline from which to compare ecological changes resulting from the revised dam operations. In autumn of 2002, the Corps began implementing the new plan, designed to be the first step in an adaptive management program.

The Corps and the Conservancy did not wait for the results of the Green River experiment before taking their partnership beyond the Louisville District. Both organizations were interested in seeing whether the same collaborative approaches applied at the Green River Dam might help them make improvements at some of the other 630 dams operated by the Corps in the United States. In 2002, the Corps and the Conservancy announced a national partnership called the Sustainable Rivers Project. General Robert Flowers, Chief of Engineers at the Corps, reported that 13 dams had been targeted for examination in the partnership, but he hoped it would soon spread to every dam they operate.

From Rivers for Life: Managing Water for People and Nature by Sandra Postel and Brian Richter. Copyright 2003 Sandra Postel and Brian Richter. Reproduced by permission of Island Press, Washington, D.C. and Covelo, California.