Upper Gila, Salt, and Verde Rivers: Arid Land Rivers in a Changing Climate

The major tributary of the Lower Colorado River, the Gila River, is a critical source of water for human and natural environments in the Southwestern US. Warmer and drier than the Upper Colorado River basin (UCRB), with less snow, and a bi-modal precipitation regime, the Gila River is controlled by a set of climatic conditions that is different from the controls on Upper Colorado River flow. Unlike the Colorado River at Lees Ferry, the Upper Gila River and major Gila River tributaries, the Salt and Verde Rivers, do not yet reflect significant declines in annual streamflow, in spite of warming trends. Annual streamflow is dominated by cool season precipitation, but the monsoon influence is discernable as well, variable across the basin and complicated by an inverse relationship with cool season precipitation in the Salt and Verde River basins. Major multi-year streamflow droughts in these two basins have frequently been accompanied by wet monsoons, suggesting that monsoon precipitation may partially offset the impacts of a dry cool season. While statistically significant trends in annual streamflow are not evident, decreases in fall and spring streamflow reflect warming temperatures and some decreases in spring precipitation. Because climatic controls vary with topography and the influence of the monsoon, the impacts of warming on streamflow in the three sub-basins is somewhat variable. However, given relationships between climate and streamflow, current trends in hydroclimate, and projections for the future, it would be prudent to expect declines in Gila River water supplies in the coming decades.

Remotely Sensed Changes in Vegetation Cover Distribution and Groundwater along the Lower Gila River

Introduced as a soil erosion deterrent, salt cedar has become a menace along riverbeds in the desert southwest. Salt cedar replaces native species, permanently altering the structure, composition, function, and natural processes of the landscape. Remote sensing technologies have the potential to monitor the level of invasion and its impacts on ecosystem services. In this research, we developed a species map by segmenting and classifying various species along a stretch of the Lower Gila River. We calculated metrics from high-resolution multispectral imagery and light detection and ranging (LiDAR) data to identify salt cedar, mesquite, and creosote. Analysts derived training and validation information from drone-acquired orthophotos to achieve an overall accuracy of 94%. It is clear from the results that salt cedar completely dominates the study area with small numbers of mesquite and creosote present. We also show that vegetation has declined in the study area over the last 25 years. We discuss how water usage may be influencing the plant health and biodiversity in the region. An examination of ground well, stream gauge, and Gravity Recovery and Climate Experiment (GRACE) groundwater storage data indicates a decline in water levels near the study area over the last 25 years.