Human Modifications

Desert streams present important challenges to understanding and management of water resources. They exemplify the resource that is most precious to humans inhabiting arid and semiarid regions, yet they are threatened by increasing pressures of human exploitation, agricultural expansion, and urbanization. Direct appropriation of streamflow to support human activities is the most serious threat to desert streams. This takes the form of diversion, interbasin water transfer, and groundwater withdrawal (which reduces base-flow); for example, groundwater withdrawals over the past century have converted the Santa Cruz River in Tucson, Arizona from a perennial to an ephemeral stream. In the Salt River of central Arizona, river diversion into a system of canals that feed agricultural and domestic/industrial demand in Phoenix has left a dry riverbed throughout the metropolitan region. Water extraction, primarily for irrigation, has also resulted in salinization of streams in much of the world and has triggered shifts in the composition of biotic communities.

In their appropriation of water for a variety of uses, people also modify the form and hence the function of desert streams. For example, the creation of canals that are straightened and lined with concrete replaces structurally complex streams with ecosystems that are unlikely to support the ecosystem functions characteristic of unmodified desert streams. Furthermore, impoundment and flow regulation can have profound effects on riparian ecosystems, for example, through the colonization and persistence of exotic plant species that outcompete native species under conditions of lowered water tables and reduced flow variability.

See also: Ecosystem Ecology; Landscape Ecology; Physical Transport Processes in Ecology: Advection, Diffusion, and Dispersion; Rivers and Streams: Ecosystem Dynamics and Integrating Paradigms; Rivers and Streams: Physical Setting and Adapted Biota; Succession.

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