Question: When did the Gavins Point Dam get damaged?
Context: The 2011 Missouri River floods was a flooding event on the Missouri River in the United States.  The flooding was triggered by record snowfall in the Rocky Mountains of Montana and Wyoming along with near-record spring rainfall in central and eastern Montana.  All six major dams along the Missouri River released record amounts of water to prevent overflow which led to flooding threatening several towns and cities along the river from Montana to Missouri; in particular Bismarck, North Dakota; Pierre, South Dakota; Dakota Dunes, South Dakota; South Sioux City, Nebraska, Sioux City, Iowa; Omaha, Nebraska; Council Bluffs, Iowa; Kansas City, Missouri; Jefferson City, Missouri, as well as putting many smaller towns at risk.  According to the National Weather Service, in the second half of the month of May 2011, almost a year's worth of rain fell over the upper Missouri River basin.  Extremely heavy rainfall in conjunction with an estimated 212 percent of normal snowpack in the Rocky Mountains contributed to this flooding event. Lookout Point Dam is an earth-type dam on the Middle Fork Willamette River in the U.S. state of Oregon.  It is located in Lane County.  Its reservoir is called Lookout Point Lake.  The dam's primary purpose is flood control, with secondary purposes of power generation, recreation, and irrigation. Green Island, 10.5 square miles of land in northwestern Cedar County, Nebraska bordering the Missouri River across from Yankton, SD, was home to Strahmburg a former town on the island.  Green Island and Strahmburg have had their names used interchangeably over the years because, in part, the post office in Strahmburg was officially designated as Green Island.  Supposedly Green Island acquired its name came the color of its lush vegetation.  Though called an island, it was physically connected to Nebraska by a swampy, low-lying area along its southern edge.  Both were destroyed by the Great Flood of 1881 on the Missouri.  Their history is better understood knowing the braided, meandering, flood-prone history of the Missouri River in that area.  Completion of Gavins Point Dam in 1957, 5 miles upriver from the former location of Strahmburg, greatly reduced the risk of floods on the river.  Today the Missouri National Recreational River, 59-miles between Gavins Point Dam and Ponca State Park, has restrictions on commercial development.  Early maps indicate the presence of numerous islands some of which, like Green Island, no longer exist. Gavins Point Dam is a hydroelectric dam on the Missouri River in the U.S. states of Nebraska and South Dakota.  Built from 1952 to 1957, it impounds Lewis and Clark Lake.  The dam is on the Nebraska-South Dakota border, west of Yankton, South Dakota.  During the 2011 Missouri River Flood the dam was damaged by debris and a significant portion of rocks were dislodged from its upstream side.  The U.S. Army Corps of Engineers soon began repairs to the dam and its spillway gates.  Pressure sensors were also installed in concrete portion of the dam. Lewis and Clark Lake is an impoundment on the Missouri River above Gavins Point Dam, near Yankton, South Dakota.  It is on the border of South Dakota and Nebraska.  The reservoir is approximately 25 mi long, with a surface area of 31,000 acres (130 km²); it has a maximum depth of 45 ft .  It is the smallest of the Pick-Sloan Plan impoundments, and has been plagued with siltation problems.  Lewis and Clark Lake (with Gavins Point Dam) is a very popular for camping as a regional tourist destination, having hosted over one million visitors per year. The Missouri National Recreational River is located on the border between Nebraska and South Dakota.  The designation was first applied in 1978 to a 59-mile section of the Missouri River between Gavins Point Dam and Ponca State Park.  In 1991, an additional 39-mile section between Fort Randall Dam and Niobrara, Nebraska, was added to the designation.  These two stretches of the Missouri River are the only parts of the river between Montana and the mouth of the Missouri that remain undammed or unchannelized.  The last 20 miles of the Niobrara River and 6 miles of Verdigre Creek were also added in 1991. Blue Mesa Dam is a 390 ft zoned earthfill dam on the Gunnison River in Colorado.  It creates Blue Mesa Reservoir, and is within Curecanti National Recreation Area just before the river enters the Black Canyon of the Gunnison.  The dam is upstream of the Morrow Point Dam.  Blue Mesa Dam and reservoir are part of the Bureau of Reclamation's Wayne N. Aspinall Unit of the Colorado River Storage Project, which retains the waters of the Colorado River and its tributaries for agricultural and municipal use in the American Southwest.  The dam's primary purpose is hydroelectric power generation.  State Highway 92 passes over the top of the dam.  Blue Mesa Dam houses two turbine generators and produces an average of 264,329,000 kilowatt-hours each year. Crystal Dam is a 323 ft double curvature, concrete thin arch dam located six miles downstream from Morrow Point Dam on the Gunnison River in Colorado, United States.  Crystal Dam is the newest of the three dams in Curecanti National Recreation Area; construction on the dam was finished in 1976.  The dam impounds Crystal Reservoir.  Crystal Dam and reservoir are part of the Bureau of Reclamation's Wayne N. Aspinall Unit of the Colorado River Storage Project, which retains the waters of the Gunnison River and its tributaries for agricultural and municipal use in the American Southwest.  The dam's primary purpose is hydroelectric power generation. Calumet Bluff is a hill about 180 feet high overlooking the Missouri River in Cedar County, Nebraska where the Lewis and Clark Expedition held its first council with the Sioux Indians for two days in 1804.  Today the Bluff forms the right or south abutment of the Gavins Point Dam.  The U.S. Army Corps of Engineers maintains the Lewis and Clark Visitor Center on Calumet Bluff next to the dam. Morrow Point Dam is a 468 ft concrete double-arch dam on the Gunnison River located in Colorado, the first dam of its type built by the U.S. Bureau of Reclamation.  Located in the upper Black Canyon of the Gunnison, it creates Morrow Point Reservoir, and is within the National Park Service-operated Curecanti National Recreation Area.  The dam is between the Blue Mesa Dam (upstream) and the Crystal Dam (downstream).  Morrow Point Dam and reservoir are part of the Bureau of Reclamation's Wayne N. Aspinall Unit of the Colorado River Storage Project, which retains the waters of the Colorado River and its tributaries for agricultural and municipal use in the American Southwest.  The dam's primary purpose is hydroelectric power generation.
Answer: The 2011 Missouri River floods