The reservoir is formed by a dam constructed by the U.S. Army Corps of Engineers on the Republican River, which starts in Colorado and ends in Kansas.

Nearly 10,000 cattle were reportedly lost along the Republican River Valley along with 300 county bridges and over 300 miles (480 km) of roads and railway.

The town moved to avoid further damage, and the U.S. Army Corps of Engineers researched a potential location for the dam would work the best to prevent such flood loss.

With a surface covering 13,250 acres (54 km2), the Harlan County Reservoir lake sits in bottom lands of the Republican River Valley.

According to the U.S. Army Corps of Engineers, the river valley got its name from the Republican Pawnee Indian tribe who inhabited the area.

Corps Staff are also responsible for the upkeeps of the roads, utility systems, campsites, beaches, boat ramps, and picnic areas in and around the lake.

The Corps also works directly with Nebraska Game and Parks Commission to manage the natural resources of Harlan County Lake.

The Harlan County Lake has fish species such as walleye, white bass, wipers, catfish, crappie and pike.

Field mice, moles, ground squirrels, rabbits, raccoons, opossum, skunk, badger, coyote, bobcat and many more rely on the habitat supported by the lake for their survival.

A strand of Cottonwood, ash, cedar, and maple trees just to name a few are found all along the Republican River and other lowland inlets leading up to the lake.

In February 2008 the Nebraska Game and Parks and U.S. Army Corps of Engineers exterminated 69 feral hogs from the Harlan County Lake area.