COMMERCE CITY, Colo. -- Years ago when Native Americans and cowboys roamed the lands in Colorado, there were millions of bison thundering across the mountains and prairies.
According to the U.S. Fish and Wildlife service, an estimated 30 million to 60 million bison roamed North America.
In the 1990s, there were only an estimated 20,000 to 25,000 wild bison with an additional 250,000 in private herds.
The only bison in Colorado are at places such as the Rocky Mountain Arsenal or along Interstate 70 near Genesse -- but none of those is wild bison.
But now there are efforts around the country to bring wild bison back.
At the Rocky Mountain Arsenal National Wildlife Refuge on Tuesday, there was the annual roundup of their herd of nearly 150 American bison.
The wildlife refuge does this every year to evaluate the herd and collect scientific data that focuses on the genetic diversity.
The goal of the program is to repopulate bison herds as genetically pure as possible.
"This herd is very important because it has a lot of the best genetics and good genetic diversity," said Nick Kaczor, the assistant manager of the refuge. "This is where bison belong. This is the kind of place they inhabited hundreds of years ago.
"They're a great tool in the fact that they replicate a lot of the things we need to restore native prairie on the landscape here at the refuge."
The bison are a 10-minute drive from downtown Denver, located near Dick's Sporting Goods Park. There's three similar refuges around the country to try to bring the bison back.