It means to travel in a north-south direction for Colorado.
I've just returned from a couple of weeks in Colorado and the American West. We made a brief stop at the
Arapaho National Wildlife Refuge Visitor Center, a parking area with a walkway overlooking the prairie. Interpretive plaques on the stonework explain the geology, fauna and flora, and history of the area. One of them said that the stream winding through the prairie below us drained into the
North Platte River, one of the few rivers in North America that runs from south to north. Looking at the images in the Wikipedia article, I see that on the first half or so of the Colorado portion of the South Platte, "downriver" is also north. Colorado rivers mostly flow south-north, west-east, or east-west.
We followed the South Platte for many miles across Nebraska. It's aptly named; "Platte" is French for
flat, and that's one flat river. It winds all over the place, with no perceptible river valley. I'm afraid that building Interstate Highway 80 right on its northern "bank" has badly inhibited its natural progress, but it's still magnificent.
You can find other, shorter south-to-north rivers in the northern part of my state of Ohio. Most of the state drains south and east into the Mississippi River basin, but the northern fifteen percent or so drains north into Lake Erie and the St. Lawrence River basin. Thus the rivers in the far north of the state generally run from south to north.