The greenback cutthroat, once widespread in the Arkansas and South Platte River drainages of Eastern Colorado and Southeast Wyoming, today occupies less than 1% of its historical range.

Within the past 20,000 years, a population which crossed the Continental Divide during the most recent ice age gave rise to the greenback cutthroat.

The species was originally thought to have been recorded in 1857 by William A. Hammond, an Army surgeon accompanying troops west during the early Indian Wars in 1857.

Two decades later, David Starr Jordan, one of the leading ichthyologists of his day, resolved the apparent impossibility by deciding that Hammond must have collected them on a trip to Fort Bridger, in the Green River basin in southwestern Wyoming, which would have likely taken him along the South Platte.

[7] Although the greenback cutthroat was considered extinct by the 1930s, in 1957 a population was discovered in Rocky Mountain National Park in the Big Thompson River, a tributary of the South Platte.

However, it was recently determined that due to insufficient study of the original stock most if not all animals in the reintroduction program were actually the similar Colorado River cutthroat trout.

[7] In 2022, Colorado Parks and Wildlife announced that the greenback cutthroat trout was discovered to be naturally reproducing having been reintroduced in Herman Gulch.