Initially called Cottonwood River State Park, it was renamed in 1945 to honor Charles Eugene Flandrau, a leading citizen of early Minnesota who commanded defenses during the Battles of New Ulm in the Dakota War of 1862.

Along with the dam, crews from the Civilian Conservation Corps (CCC) and Works Progress Administration (WPA) built several structures in the National Park Service rustic style.

In a unique twist on the mandate to harmonize with the local environment, the buildings were designed to reflect the ethnic German heritage of New Ulm.

The valley floor supports marshes and wet prairie interspersed with bottomland hardwood forest of willow, eastern cottonwood, American elm, silver maple, and green ash.

Without periodic wildfires to thin woody plants, the valleyside forests have gotten denser and many prairie openings have filled in with sumac.

[4]: 34  The 25 species of mammals seen in the park include white-tailed deer, coyotes, gray foxes, raccoons, beavers, skunks, opossums, and minks.

[4]: 39 Archaeological evidence found outside the park confirms that prehistoric Native Americans inhabited the Cottonwood River valley.

Mostly middle-aged skilled workers, the WPA men quarried stone and constructed three park buildings: a beachhouse, a manager's residence, and a garage.

They built the dam and its earthen dikes, constructed a kitchen shelter, landscaped, planted trees, and blazed trails.

In July 1936 they transferred to other Minnesota state parks and were replaced with the unemployed young men more typical of CCC projects.

[9] The stone structures built by the WPA have been called " the most unusual architectural designs in the state park system" by the Minnesota Historical Society.

In this case, however, architect Edward W. Barber chose to reflect local culture as well, honoring the strong German heritage of New Ulm.

The park buildings evoke the architecture of Germany with steeply pitched roofs, dormers, chimneys, casement windows with small panes, and fine nonlinear stonework.

Late in the war, Great Britain was short on space and resources to provide for captured enemy combatants; meanwhile, much of the U.S. labor force was off serving in the military.

Nine POW camps were established in Minnesota — including one other state park, Whitewater — all managed from a regional headquarters in Algona, Iowa.

That year their use on farms expanded considerably, encompassing worksites in eight counties, while prisoners at the cannery were instrumental in packing Sleepy Eye's largest-ever pea crop.

[12] German-speaking church officials held Lutheran and Catholic services in the camp and gathered donations of reading material.

[13] Although the guards warned civilians that they were not to have contact with the POWs, food was slipped over the fence, cannery workers shared ice cream and beer, and young women waded across the river at night to flirt at the camp's edge.

[11][12] POWs out on weeklong farm details fared best of all, often receiving full home-cooked meals at the family dinner table.

[10] By World War II, sentiment developed for renaming the park after Charles Flandrau (1828–1903), a notable figure in early Minnesota history.

The engineering consultants reported that, although a restored dam would improve habitat for panfish and block upstream movement of undesirable species such as carp, it would not provide effective flood control.

[4]: 57  The capacity of the lake basin was too small compared to the watershed, exacerbated by increased agricultural drainage and wetland loss since the original damming in the 1930s.

Nor would a new dam be cost-effective for generating hydroelectricity, and the lake would be prone to heavy sedimentation and periods of poor water quality.

[7] Some structures remained for several years; full dam removal took place in 1995 and the Cottonwood River is again free-flowing through the park.

[4]: 39 Flandrau is adjacent to Nehls City Park, the private New Ulm Country Club, and the August Schell Brewing Company.