Though geologically complex, overall the land surface of Arizona tilts slightly downward to the southwest. More than 90% of the state's drainage flows into the southwest corner via the Colorado River and its tributaries. By the time the river enters Mexico, it has descended to an elevation of only 70 feet. Mountain ranges rise in nearly every part of Arizona, but they achieve their greatest heights in the north-central and eastern sections. Humphrey's Peak, part of the San Francisco Peaks near Flagstaff, crowns the state at 12,633 feet. Geographers divide Arizona into the high Colorado Plateau Province of the north and the Basin and Range Province of the rest of the state. Average elevation statewide is about 4,000 feet. Measuring 335 miles wide and 390 miles long, Arizona is the sixth largest state in the country.

Colorado Plateau
This giant uplifted landmass in northern Arizona also extends across much of adjacent Utah, Colorado, and New Mexico. Rivers have cut deeply into the plateau, forming the Grand Canyon of the Colorado and other vast gorges. Volcanoes have broken through the surface and left hundreds of cinder cones, such as multicolored Sunset Crater. The most recent burst of volcanic activity in Arizona took place at Sunset Crater about 800 years ago. Most elevations on the plateau are between 5,000 and 8,000 feet. Sheer cliffs of the Mogollon (MUGGY-own) Rim drop to the desert, marking the plateau's south boundary. To the west, the plateau ends at Grand Wash Cliffs.

Basin and Range Province
Many ranges of fault-block mountains, formed by faulting and tilting of the earth's crust, poke through the desert plains of western, central, eastern, and southern Arizona. Several peaks rise above 9,000 feet, creating biological islands inhabited by cool-climate animals and plants. Tucson-area residents, for example, can leave the Sonoran Desert and within an hour reach the cool fir and aspen forests of Mt. Lemmon.


During any season, some part of Arizona enjoys near-perfect weather. Sunny skies and low humidity prevail over the entire state most of the time. Average winter temperatures run in the 50s F in the low desert and the 20s to 30s F in the mountains and high plateaus. Desert dwellers endure average temperatures in the 80s and 90s in summer, when high-country residents enjoy averages in the 70s F. Parker, along the Colorado River, attained 127F on July 7, 1905—the highest reading ever recorded in the state. Even on a normal summer day in the low desert, you can expect highs in the low 100s. Desert areas can experience swings of 40F between day and night due to the dry air and the lack of moderating forests.

Rain and snowfall correspond roughly to elevation: The southwest corner receives less than five inches of precipitation annually, while the higher mountains and the Mogollon Rim get about 25 inches. Most falls either in winter as gentle rains and snow, or in summer as widely scattered thundershowers. Winter moisture comes mostly in December through March, revitalizing the desert; brilliant wildflower displays appear after a good wet season. Summer afternoon thunderclouds billow in towering formations from about mid-July to mid-September. The storms, though producing heavy rains, tend to be localized in areas less than three miles across. Summer thundershowers make up 60-70% of the annual precipitation in the low desert and about 45% on the Colorado Plateau.

Storm Hazards
Rainwater runs quickly off the rocky desert surfaces and into gullies and canyons. Flash floods can form and sweep away anything in their paths, including boulders, cars, and campsites. Take care not to camp or park in potential flash-flood areas. If you come to a section of flooded roadway, a common occurrence on desert roads after storms, just wait until the water goes down—usually within an hour or so. Summer lightning causes forest and brush fires, and it poses a danger to hikers foolish enough to climb mountains when storms threaten.

On to Flora and Fauna