There actually is a pump + tunnel system connecting the Colorado and Mississippi river systems, though it runs the other way around, bringing water from the Col

There actually is a pump + tunnel system connecting the Colorado and Mississippi river systems, though it runs the other way around, bringing water from the Colorado river drainage across the mountains to the east: https://en.wikipedia.org/wiki/Alva_B._Adams_Tunnel https://en.wikipedia.org/wiki/Alva_B._Adams_Tunnel The problem is that even once you cross the mountains to the Mississippi catchment, there isn't much water on the east side for hundreds of miles. If you wanted to get water from the Mississippi River itself all the way west to the Rockies, you'd have to pump it about a mile uphill just to get it to the Denver area.