Would really need to see a leaf and the part of the stem where the leaf joins the stem. Also, you have posted this question on the "native" plant forum, but not all plants found in the wild are native. You may want to post this on a broader forum such as the "name that plant" forum.

I would look at the site linked below. Does it have spines or thorns? Looks like it may have flowered and has fluffy seedheads. Russian thistle?

My suggestions are :
http://www.co.larimer.co.us/publicworks/weeds/30weeds/dflax.htm

http://www.co.larimer.co.us/publicworks/weeds/30weeds/kochia.htm

http://images.google.com/images?hl=en&q=Salsola%20iberica&um=1&ie=UTF-8&sa=N&tab=wi

Here is a link that might be useful: 30 weeds in Colorado

also, please rub the leaves and determine if it has a scent

MaybeSt. Johnswort? check the link below

Here is a link that might be useful: Common weeds of Colorado

Looks kind of like alfalfa.