I found this spider yesterday running across a sidewalk next to a canal,
it was bigger than a quarter and I think it was carrying an egg sack.
Does anybody know what kind of spider this is?
The white ball is an egg sac. From Wikipedia:
wolf spiders carry their egg sacs by attaching them to their spinnerets.
Spiderid.com suggests this species has been spotted in Colorado previously:
Hogna frondicola has also been sighted in the following states: Colorado, Maine, Montana.