The Rainwater Basin wetland region is a 4,200 sq mi (11,000 km2) loess plain located south of the Platte River in south-central Nebraska.

Before European settlement, this plain was covered by prairie grasslands interspersed with thousands of ephemeral playa wetlands, called Rainwater Basins.

The shallow depressions, in which these wetlands occur are lined with a nearly impervious layer of clayey soil, a claypan, that prohibits surface water from penetrating the subsoil.

All of the federally managed land was acquired from willing landowners and purchased with the proceeds of duck stamps that are sold to hunters each year.

Most basins are closed depressions that hold runoff from the surrounding landscape to form seasonal lakes and wetlands in them.

Many of the large semi-elliptical to elliptical Rainwater Basins have a crescent-shaped ridge that Stark referred to as a lunette located on the southeast side of them.

Where they have been cored, the ridges underlying the loess consist of well-sorted sand on the order of 3–5 meters (9.8–16.4 ft) thick.

[15][16] The fluvial sands of a pleistocene Platte River, which overlain by the Gilman Canyon formation and younger loesses, yielded optically stimulated luminescence dates from about 36,000 to 50,000 BP calendar years.

These dates demonstrate that the Rainwater basins were formed prior to the Middle Wisconsinan, Marine Isotope Stage 3, and the accumulation of the Gilman Canyon Formation.

[13][14] Thus, Rainwater basins and their associated ridges are relict, pre-Last Glacial Maximum landforms that have migrated upward in elevation as successive layers of glacial-age loess have accumulated and intercalated soils sporadically formed.