Modeling Land Use and Climate Change Effects on Hydrogeomorphology and Cottonwood-Willow Distribution in the Verde Valley, Arizona

The Verde River has changed over the past several decades. Channel width has increased and stands of Fremont cottonwood (Populus fremontii) and Goodding willow (Salix gooddingii) have declined noticeably since the mid -1980s in the Verde River greenway. The greenway is a designated area within the town of Cottonwood, which is managed cooperatively by Arizona State Parks and local landowners. Although increasing flood magnitudes due to decadal -scale climate change (Redmond et al. 2002) may be primarily responsible for these changes, as discussed later in this paper, a question remains regarding the role of human activity, as river corridor changes have coincided with the urbanization of what was formerly an agricultural landscape. Hydrologic and geomorphic processes strongly influence riparian tree distribution in the active channel area (Bendix and Hupp 2000; Stromberg 1998). Riparian vegetation is particularly affected by flood processes that shape fluvial landforms (Bendix and Hupp 2000). In arid regions, fluvial landforms tend to be transitory, subject to frequent and rapid change (Graf 1988). The cottonwood willow distribution in the Verde Valley changed dramatically from 1977 to 1995 as floods of increasing magnitude over time (Figure 1) destroyed many mature trees, scoured and deposited sediment, and produced substrate characteristics that promoted germination and growth of new cohorts (Masek Lopez and Springer 2002). Although geomorphic thresholds in rivers determine morphology and habitat, it is important to note that human activity can precipitate threshold crossings and these activities frequently dictate the character of the riverine landscape, including riparian tree distribution (Church 2002). Because climatic changes have coincided with increasing urbanization in the