Through this segment and well into the next the Colorado River and its geologic consequences will be our companion, including the arresting Glenwood Canyon alignment which we begin here and continue into Part 12. The Colorado River, the principal river of the southwestern United States, runs 1,450 miles from its headwaters at La Poudre Pass in the central Rocky Mountains (10,184'), at the Continental Divide, to its mouth at the Gulf of California between Baja California and Sonora in Mexico. Among its many tributaries, some defined officially, include the Gila River, the Gunnison River, the Little Colorado River, the San Juan River, the Dolores River and of course the Green River, passing through Glenwood Springs, Grand Junction, Moab (UT), Lake Havasu City (AZ), Yuma (AZ) and San Luis Rio Colorado (Sonora MX). At its greatest discharge it pours a torrential 384,000 cubic feet per second from its 246,000 square mile basin.

Modern geology believes the Colorado River first formed as a westward stream after the geologic upheaval that thrust up the Rocky Mountains during the Laramide Orogeny, approximately 50 to 75 million years ago (MYA). The same uplift redirected the Green River, its major tributary, from its flow towards the Mississippi instead to the Colorado. Before the Gulf of California formed approxmiately 5-10 MYA, the original course of the River drained possibly as far north as modern Monterey, CA. (A remnant of this ancient course is the submerged Monterey submarine canyon.) The formation of the Sierra Nevada range around 4.5 MYA forced the Colorado southward, and as the Colorado Plateau increased in height simultaneously, the new course of the river dug a deep gouge which today is known as the Grand Canyon. Its large delta in southern California, Arizona and northern Mexico included the geologic depression known as the Salton Sink, yielding Lake Cahuilla from 1000 AD to roughly 1500 or 1700 AD, which last flooded in 1905 to yield the Salton Sea.

At its historic peak the River entered its ancient delta with a discharge as high as 22,500 cubic feet per second, substantially varied by rainfall and snowmelt runoff. From 2 MYA to as recently as 10,000 years ago, basalt flows from the Unikaret volcanic field in northern Arizona repeatedly dammed the river within the Grand Canyon and formed at least 13 lava dams. Most of these dams collapsed, yielding some of the largest floods ever to occur in North America likely exceeded only by the Missoula Floods in Oregon and Washington. The chronic flooding risk along the Colorado River was likely catastrophic to early peoples inhabiting the valley, who nevertheless later formed some large tracts of agriculture that lasted centuries. Primitive irrigation canals showed humanity's interest in taming the great river even then, particularly the Hohokam, who built an extensive network ranging as long as 300 miles total. Drought, however, was what ultimately caused these early cultures' collapse, somewhere around AD 1300; the Navajo, who migrated to the valley around AD 1000, were able to survive on skills acquired from their predecessors, along with the Mohave who fished in the relatively plentiful lower Colorado and cleverly used the flooding to irrigate their fields, and the Utes, who gradually extended south around AD 1500. Conflict was inevitable when the white man arrived, first with the Spaniards in the 1700s, and then an American influx which was cemented with the 1848 Treaty of Guadalupe Hidalgo. Among other peoples, the federal government eventually forcibly relocated the Navajo, Mohave and Utes, and subsequent treaties at gunpoint established the modern-day reservations, most notably the Navajo Nation, the largest Indian reservation in the country.