The environmental flows approach to water management is one way to incorporate ecological knowledge into water use decisions. The term “environmental flows” refers to the amount, timing, and quality of water required to maintain aquatic and riparian ecosystems. In the semi-arid American southwest, water is often managed with little regard for ecological consequences, resulting in dewatered rivers, lowered water tables, and altered riparian species composition and ecosystem processes. By determining the hydrologic needs of aquatic and riparian ecosystems and reserving the appropriate amount of water as instream flow, anthropogenic water needs may be accommodated while causing minimal ecological impact.

The Verde River is a large, perennial, free-flowing river in central Arizona. Proposed groundwater pumping in the Verde River watershed may dewater the river’s upper 26 miles, which are spring fed. As part of a larger project to determine environmental flows for the upper Verde River, we have developed flow-ecology relationships for common woody riparian species and riverine marshland. These flow-ecology relationships are based on data collected from study sites along the Verde River combined with eco-hydrologic knowledge of the species and plant communities from other regional rivers, including the San Pedro River, Hassayampa River, and Cienega Creek.

Results/Conclusions

Data from the Verde and other regional rivers suggest that width of riverine marsh declines with summer base flow levels and annual stream flow, and that plant species richness and relative cover of hydric wetland plants within the active channel zone are greatest at sites with perennial flow. On other regional rivers, active channel species richness and cover of hydric perennial species declined as streams became more intermittent, suggesting the same would occur if the upper Verde were dewatered.

Depth-to-water table relationships for riparian woody species common on the Verde River suggest the species most sensitive ground water declines are the hydric pioneers Fremont cottonwood and Goodding willow and the mesic species velvet ash. Data from other regional rivers corroborates the sensitivity of Fremont cottonwood and Gooding willow to ground water decline; these species occur on geomorphic surfaces that are at an average of 2 meters above the water table. Knowledge of Verde River riparian vegetation water requirements may aid resource managers in accommodating ecosystem water needs when making important management decisions.