Book Challenges Perception of Vast Demise of SW Riparian Wetlands

The Ribbon of Green: Change in Riparian Vegetation in the Southwestern United States
Robert H. Webb, Stanley A. Leake and Raymond M. Turner. University of Arizona Press, $75 cloth. For information about ordering check: www.uapress.arizona.edu

The loss of riparian areas in the Southwest after years of human settlement is generally figured to be great, with the loss variously reported from 80 to 95 percent. These figures often appear in the news media and popular and scientific literature and underlie various laws and regional management plans.
A recently published book, “The Ribbon of Green: Change in Riparian Vegetation in the Southwestern United States,” looks at long-term changes in woody regional riparian areas and questions the occurrence of a precipitous riparian loss. The authors identify a paper they believe was the source for the 90 percent figure that got widely circulated.

In their study, the authors, hydrologists Robert H. Webb and Stanley A. Leake and botanist Raymond M. Turner, focus on a geographic area that includes the major river valleys in parts of Utah, southern Nevada, and southeastern California as well as all of Arizona below above 5,000 feet in elevation.

Repeat photography combined with a review of historical context and information on species composition enabled the researchers to document the condition of riparian vegetation during the last 140 years in the Southwest, a time period ranging from the first use of the camera to the present. The authors studied about a dozen woody species and various herbaceous perennials visible in photographs. To evaluate spatial changes the authors analyzed aerial photography or satellite images available in the last quarter of the twentieth century.

Changes observed in thousands of repeat images were interpreted along with surface water and groundwater hydrologic data, previous periods of climatic variation, land uses and flow regulation, and water usage. In examining the factors affecting the stability of woody riparian vegetation, the authors considered the diversion of surface water, flood control and the excessive pumping of groundwater.
The authors challenge the popular assumption about the vast decline of Southwest riparian wetlands. They make the case that rather than a 80 or 90 percent loss, wetland vegetation has actually increased on many river stretches in the region. This has been due to flood control, favorable climatic conditions and large winter floods that have encouraged ecosystem disturbance, germination, and the establishment of species in newly generated openings.

The authors consider various perceptions that have guided researchers’ views of long-term change in the region’s rivers. They acknowledge that some scientists perceive humans as ultimately causing regional change, a view prompting them to interpret all changes as bad. Others scientists give more weight to climatic fluctuations as a major influence on rivers of the region. The authors consider that both perceptions are parts of the truth with local effects an added complexity.

The book’s contribution to the ecological study of wetlands is the broadening of our understanding of change in riparian ecosystems. This in turn will affect riparian restoration strategies.

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