ARTIFICIAL HYDROLOGIC CONTROLS AND THE GEOMORPHOLOGY OF THE GREATER PHOENIX AREA

The way in which humans rearrange and utilize the landscape is a powerful geomorphic force. The greater Phoenix area is well suited to study the effects of artificial hydrology and geomorphology. As one of the fastest growing cities in the United States and inhabited by nearly three million people in a semi-arid region, water here is a highly valued and managed resource. We address two questions: 1) to what degree do artificial controls on hydrologic processes alter those flow systems, and 2) how do these changes affect urban geomorphology?

There are a variety of reasons for which any city alternately draws to and diverts water away from a location – these are primarily to deliver water or divert floodwater. In Phoenix, engineered solutions to these cultural pressures include groundwater pumping and agricultural irrigation, the construction of residential and industrial buildings, flood prevention structures, mining operations, canals, dams, washes, and the sustaining of aesthetically lush areas.

We have looked at a few specific areas in answering these questions. Cave Creek is an increasingly developed watershed, running from 40 km north of Phoenix into the historically oldest part of the city. The overland flow through this area becomes increasingly managed downstream, as it is constrained from its natural network into a single volume-restricted channel and a series of retention basins. It is then diverted into the adjacent Agua Fria River to reduce the watershed size and thus the geomorphic impact on urban areas downstream of the original channel.

Another example is that of Indian Bend Wash, constructed in 1974. This flood control structure was once an ephemeral desert wash, but due to human geomorphology it is today a string of perennial lakes. The artificial control most evident here lies in the geometry of the flow channel and the prevention of natural erosion processes.

Finally, the Hohokam Native Americans who lived on the Salt River (South Phoenix and Tempe) built a system of irrigation canals, but still experienced natural geomorphic processes like flood events. Today, in response to urban expansion, dams and other flood control structures throughout the watershed have essentially dried up the flow of the Salt River. It is diverted instead into a network of canals, delivering water far outside the natural floodplain.