Juliet Stromberg, who's also a life sciences associate professor, said since she began monitoring the river at a series of sites in 1993 that she has seen the decrease in stream flow and groundwater levels result in changes to plant and animal life in the ecosystem.

"There is reason to be concerned. We're documenting a decline in base flows. Part of the river is becoming intermittent that was perennial," she said, meaning fewer portions of the river run all year long.

In response to changes in stream flow permanence and the depth of San Pedro River ground water, cottonwoods, tall high-canopy willow trees that fill the valley, are being replaced by other, shorter and more shrub-like species that require less water, Stromberg said.

"As the water level drops, ecology fluxes ... it's transitioning from a forest structure to a scrubland."

Gary Woodard coordinates water and energy conservation efforts as part of the UA Sustainability of Semi-Arid Hydrology and Riparian Areas organization.

"Once the flow becomes intermittent, far fewer plants can be supported. Shelter, nesting places for birds, start to see a real drop off in diversity including fish, birds and insects," he said.

As one of the last rivers left in southern Arizona, the San Pedro is a major migratory corridor for hundreds of bird species. More than 400 species of birds have been found in the river valley, which hosts an estimated 1 million to 4 million birds annually during migration.

As habitat patches become fragmented due to the weakened system, existing habitats are declining in quality, Woodard said.

"We know what happens, for example in Tucson and Phoenix. Water tables have been lowered and the riparian ecosystem have dried up."