Irrigation observations..

With the bad weather behind him, Mark made good progress yesterday and finished the day heading for the Interstate 10 and the border with NewMexico. At this point it is probably worth taking a wider view again and putting his current location into a broader context. ...
Mark is now crossing the southern end of the Rockies, the mountain range which dominates the physical geography of the western half of the United States. Technically, they are 'young' (65-100 million years old) folded mountains which are crumpling up as the Pacific Plate collides with the North American Plate. The Rockies reach their highest altitide (just over 4000m) in the state of Colorado. In Arizona, the Rockies produce the highest relief in the east of the state at Bald Peak (3400m). Mark's route through the mountains, however, takes him south of this and for much of yesterday exploited the lower ground of the Gila River valley. The Gila valley stands out clearly from the air as an irrigated line in the arid surrounding mountains. The river Gila is a 1000km long tributary of the Colorado River and it should be one of the largest desert rivers in the world. However, abstraction of water for irrigation and municipal use turns it into a largely dry river, particularly west of Phoenix. Incredibly , before it was 'managed', it was 350m wide at its widest and 12 deep at its deepest and was navigable almost to the New Mexico border! However, for much of its length today, it looks like this...

Climbing out of Globe yesterday morning Mark passed close by one of the major water management schemes on the Gila River - the Coolidge Dam. In this image the Gila river flows east to west and is joined from the north by the San Francisco tributary shortly before entering the lake formed hehind the dam constructed at its western end. (The image is slightly deceptive, as at first you are tempted to imagine that the water flow is in the opposite direction i.e. that the lake provides the water to irrigate the land to the east). The dam, seen above in an Flickr image, holds back the waters of the San Carlos lake which are periodically released along this channel to provide irrigation water downstream towards Phoenix. This part of Arizona receives only in the region of 150 - 200mm of rain annually, which, combined with high rates of evaporation would make cultivation impossible without irrigation. The green 'ribbons' on the maps above reveal the importance of abstracted river water for the communities beside the stretch of the Gila river along which Mark cycled yesterday. So what crops are cultivated here? Well, it would appear that they include winter lettuce, cotton (trivia fact... Arizona produces enough cotton annually to make a pair of jeans of every US resident!) , melons of all kind and also a range of vegetables including spinach, broccoli and cauliflower.
South of the Gila valley, the land rises steeply to the summit of Mt Graham, location of the Mount Graham International Observatory . Constructed in 1988 , Mount Graham was selected from a survey of 280 potential mountain sites on the basis of astronomical considerations such as clear skies, low light pollution, low atmospheric water vapor, and ease of access. In geography, we often consider location factors but it is generally in the context of settlement or industry. Location factors for an astronomical observatory make a change! The location of the observatory is, however, a source of considerable controversy particularly with regard to local Apache claims to the land.
Finally, just a few kms west of Marks' campsite last night, some now familiar 'spots' - circular areas of pivot irrigation which we have seen in many places along Mark's route from Iran to USA . They certainly make an interesting pattern of land use when viewed from the air.