Colorado River Watershed - Lower Basin
The upper basin of the Colorado River Basin covers the lands of Utah, Colorado, and Wyoming. In campaign terms, these lands are thickly infested by powerful giants and dragons. There are no large scale civilizations in that area, although scattered communities of settlers can be found, of course.
The lower basin begins officially on the south side of Lake Powell, as the Colorado flows westward through northern Arizona. The far right/east side of the basin is formed by the mountains along the Arizona/New Mexico border. The peak of that mountain ridge is actually in New Mexico, and the headwaters of the Gila begin there.
The state is divided across the middle by a major ridge called the Mogollon Rim. North of the rim, the water flows to the Little Colorado River (which joins the Colorado before the Grand Canyon). South of the Mogollon Rim, the waters flow south into two rivers: the Salt River on the right, and the Verde River on the left.
The Verde joins the Salt in the northeast Phoenix Valley, producing a fairly large river that flows through the desert valley from top right to lower left.
There is also a mountain ridge in the southeast part of the state. The San Pedro River and the Santa Cruz River flow northward from those mountains, to the Gila River. The desert washes of southern Arizona all flow north into the Gila, though they are usually dry.
The Salt eventually joins the Gila in the southwest Phoenix Valley. The Gila flows around and through various mountains on its way southeast to join the Colorado at Yuma.
The main human settlements are situated along the Gila and the Salt rivers, which run strong but shallow through the desert, allowing for large-scale irrigation. The largest of the towns is Tempe, where they king and his court reside. Smaller farming towns include Mesa along the Salt, Coolidge along the Gila, and Buckeye after their confluence.
The Verde River is largely cut through steep canyons, so is not generally suitable for large-scale crop irrigation. The exceptions are Cottonwood and Camp Verde, which are two farming communities located along some of the few flat planes along the Verde River banks.
The towns of Prescott and Flagstaff get most of their food supply from the agricultural areas along the rivers. Those mountain towns provide the raw metals and timber for the farming towns. Thus, the Highway of the 17 Lords (I-17) connecting the Phoenix Valley to Flagstaff, is an crucial economic artery and the main highway in Arizona.