I’m an easterner and I’ve never spent a lot of time on squirrel taxonomy. Where I come from we have tree squirrels and we have chipmunks. Because of that bias, I assumed the critters above were chipmunks. They’re actually antelope ground squirrels (Ammospermophilus sp., Sciuridae). The most obvious difference is that the stripes on chipmunks extend over their bodies and faces, while ground squirrels only have stripes on their bodies (if at all).
This photo was taken near an overlook/picnic area just north of Hoover Dam. A rocky hill ran down to the edge of Lake Mead, and there were a lot of these squirrels scurrying around. I think they were scavenging for food thrown down by generous humans (though you shouldn’t feed the !@(*&# wildlife).