My go to rods for fishing the Colorado River, lakes Martinez, Ferguson, Squaw, and the side channel lakes on the Colorado river are 8 and 9wt rods. I use the 8 most often. Sometimes a 6 or 7 for panfish or carp in open water. You will often be fishing either in current in the river or around heavy structure in the lakes and river. Stripers, LM bass, SM bass, cats, carp, panfish. An 8 or a 9wt will cast a heavier line which will help when you want to cast heavier flies or handle a heavy 300 to 350 grain sinking line in the river. The 8 would also be useful if you make it up to Havasu or Mead. For casting bass bugs or mice I typically use an 8wt rod with a 9 or 10wt floating lines. Im not usually making tremendously long casts so the heavier line works well for tossing bulky bugs. Oh, I have seen plenty of carp that would go over 30lbs. Plenty of 3-5 lbers too. If I'm fishing canals I will use a 6 or 8 wt depending on water flow in the canals and the size of the fly I'm using.
If I was only going to have one rod over there it would be an 8wt.