A ferocious debate that has raged for more than three decades over the fate of a river in the Sierra Nevada foothills appears likely to end soon with a whimper. Within the next few months, the U.S. Bureau of Reclamation (USBR) expects to conclude its environmental review and begin seeking a contractor to build a gate of steel plates across the mouth of a diversion tunnel bored through a ridge in the American River canyon northeast of Sacramento. When that gate closes — probably within two years, according to Jeff McCracken, a USBR spokesman — the river will return to its natural course through the long-idle construction site of the Auburn Dam. By agreeing to block the 2,400-foot diversion tunnel and restore the river to its channel, the USBR likely has sounded the death knell for what would have been the biggest concrete dam in the United States at 800 feet high and 4,000 feet wide at its crest. The agreement also means a new era in flood-control planning for the Sacramento Valley. Founded more than 150 years ago in the floodplain at the confluence of the American and Sacramento rivers, California's capital city is protected today by a system of levees, bypass channels and dams. The most prominent of these protective structures is Folsom Dam, completed in 1957 about 25 miles upstream from the Capitol. Folsom is a multipurpose dam designed to generate electricity, hold back floods and impound a water supply for farms and cities. Its effectiveness is hampered, however, by the contradictory natures of these tasks. To capture spring floods, the reservoir behind the dam must be drawn down during the winter, when demand for irrigation water and electricity are low. If spring runoff is sparse, however, the reservoir will greet the beginning of the dry season with depleted storage. But if the dam's operators hold back too much water, they will not have room to capture the runoff from a giant storm. Folsom's operational shortcomings are exacerbated by its designers' unwitting reliance on inaccurate estimates of the river system's flood potential. Folsom was designed to protect Sacramento from a 250-year storm, but floods in 1955, 1963 and 1965 demonstrated that the hydrologists' estimates of potential runoff were far too low. When the federal government subsequently redrew its floodplain maps of the area, thousands of residents — and numerous proposed developments — became subject to flood insurance requirements and building restrictions. To many Sacramento-area politicians, business owners and community leaders, the solution to the city's floodplain woes was another dam upstream from Folsom. Congress authorized Auburn Dam in 1965 and the USBR began construction in 1967. In 1975, however, an earthquake struck about 45 miles away near Oroville Dam. At magnitude 5.7, the quake was five times more powerful than Auburn Dam had been designed to withstand. Construction halted while geologists conducted new seismic surveys and engineers redesigned Auburn, which was being constructed directly atop a fault. Work never resumed. Instead, the project fell victim to rising costs (estimates of which exceeded $1 billion), public concern about quake safety, opposition by the increasingly powerful environmental movement and the growth of a lucrative recreational rafting industry. Auburn Dam would drown more than 40 miles of the Middle and North Forks of the American River, an area that records more than 500,000 visitor days per year, according to the California Department of Parks and Recreation. Still, Sacramento politicians and business leaders maintained their faith in Auburn Dam, encouraged by a destructive flood in 1986 that nearly brought disaster to the capital. Led by Republican Rep. John Doolittle, who has fought doggedly for the dam for more than a decade, the region's congressional delegation pressed hard for funding. But their bills failed on the House floor in 1992 and in committee in 1996. The delegation fragmented after that, with Democrat Rep. Robert Matsui and Republican Rep. Doug Ose throwing their support behind plans to improve levees and raise the height of Folsom Dam by 7 feet. The Senate Appropriations Committee in July approved initial funding for a study of the Folsom project, which has been endorsed by the U.S. Army Corps of Engineers and the Sacramento Area Flood Control Agency. The USBR began considering closing the tunnel bypassing the moribund dam site after California Attorney General Bill Lockyer sent federal officials a letter in 1999 warning that they had a public-trust obligation to restore the American River, and threatening litigation if they did not do so. In March 2000, the USBR and the state began negotiating a deal, which they signed in January 2001. The tunnel closure will follow construction of a permanent pumping station for the Placer County Water Agency (PCWA), enabling it to withdraw up to 35,000 acre-feet from the river each year. (The bureau had removed the PCWA's original pumping station when dam construction began in 1967, providing temporary pumps so PCWA could withdraw as much as 25,000 acre-feet annually.) The USBR released the final environmental impact statement for the project on June 20. Once the river reoccupies its original channel and begins supporting fish and wildlife, odds are slim that anyone will be able to force it out again, McCracken said. Laws passed since the dam's 1965 authorization — the Clean Water Act, the National Environmental Policy Act and the Endangered Species Act among them — would hamper efforts to once again divert the river's entire flow. Doolittle, whose suburban district needs water more than flood control, remains convinced that Auburn Dam should be built. He argues that the federal government will have spent almost as much on the Folsom project and levee improvements as it would have to build the dam, without providing nearly as much protection. "As long as there are people living in harm's way there will be a need for Auburn Dam," said Richard Robinson, Doolittle's press aide. To spend millions of dollars on projects that will not assure Sacramento's safety from a catastrophic flood, he said, "is a misuse of federal funds." To opponents of the dam, however, the USBR's plan to block the tunnel is reason to rejoice. "We talk of it as putting another nail in the coffin of Auburn Dam," said Betsy Reifsnider, executive director of Friends of the River, which has been battling the project almost since its inception. "I think that when they put the water back in that river, there will be lots of people watching from the bank with tears in their eyes." Contacts: Jeff McCracken, U.S. Bureau of Reclamation, (916) 978-5100. Rep. John Doolittle, (202) 225-2511. Betsy Reifsnider, Friends of the River: (916) 442-3155.