The Colorado River is the seventh-largest river in the United States, and its basin spans seven U.S. and two Mexican states. It currently supplies water to 50 million people with an expected increase of 23 million by 2030. Because it winds its way through several different regions, the policies regulating it are complex, but without close management and monitoring, demands on this water source could outweigh its supply by 2060.
To address these concerns, Riley Swanson, a recent master’s graduate of geology who studied under the direction of Abraham Springer, professor in the School of Earth and Sustainability, led the study titled, “Quantifying the base flow of the Colorado River: its importance in sustaining perennial flow in northern Arizona and southern Utah (USA)” published in Hydrogeology Journal.
“This research deals with water, a crucial resource in high demand and often over-allocated in the West,” Swanson said. “It looks to highlight the amount of groundwater that feeds the Colorado River, a water source for more than one-seventh of the U.S. population.”
Originally from Fort Collins, Colorado, Swanson attended Pacific Lutheran University in Washington, studying glacial geology in the alpine glaciers of Mount Rainer. He graduated in 2015 with a bachelor’s of science in geoscience and a bachelor’s of art in environmental studies. Afterward, he worked as a recreational guide for several outdoor sports, which he said was the catalyst for pursing higher education at Northern Arizona University.
When he arrived at NAU, his focus shifted to hydrogeology, specifically groundwater and its contributions to rivers—an important contributing factor to maintaining perennial, or year-round flow, but one often overlooked in the management practices of the Colorado River. Swanson’s team said the oversight may be due to contrasting results of previous base-flow studies. Base-flow is the amount of stream flow—water running in the river—sourced from groundwater.
Some studies dismiss groundwater contributions altogether, while others claim it contributes to a majority of stream flow. This study connected the two by looking at the interaction between groundwater and surface water.
“The key finding of this study is the quantity of groundwater contributing to the Colorado River from the study area of the greater Grand Canyon region, a region often overlooked for any contribution,” Swanson said. “However, this area contributes a substantial amount of groundwater, totaling over three percent of the mean flow of the Colorado River. This highlights the importance of groundwater contribution to the Colorado River.”
Ten remote subbasins of the Colorado Plateau in southern Utah and northern Arizona were studied, selected for the little disturbances they have to their hydrologic systems. New and extant instrumented data (direct methods) were used to quantify the groundwater base-flow contribution to the flow of the Colorado River.
Previous remote sensing data has shown the groundwater storage trend as declining. The instrumented data in this study showed the average annual base-flow trend of the area as constant, suggesting base-flow signatures in streams may have delayed responses from the decline observed from remote sensing.
“What stood out to me the most was the disconnect of this study’s findings on base-flow compared to regional groundwater findings,” Swanson said. “This study found no statistically significant declines in base-flow contribution to the Colorado River. However, regional studies with remote sensing techniques utilizing GRACE data, which is repeat satellite gravity surveys monitoring basin-wide water storage changes, have found large declines in total groundwater storage. We attributed this disconnect to a potential delay in the response of groundwater storage loss to observed trends in base-flow.”
The results of the study indicated that groundwater should not be viewed as an additional water source, but rather a source assessed on how it contributes to surface flow—groundwater supplies surface flow.
Water usage restrictions typically address surface flow, but if groundwater is used to supplement surface flow needs, it will lead to an overall decrease in water supply, both surface flow and groundwater. This could lead to aquifer compaction, increased pumping costs, ground subsidence, harm to dependent ecosystems and overall negative consequences in discharge to springs and rivers.
“This study took a large amount of new and extant data and was able to bring it all together to tell a story,” Swanson said. “This synthesis allowed for a more robust estimation of base-flow to be made for an area with limited previously published data.”
“Riley developed some innovative analysis techniques to quantify the base-flow of springs and streams in remote regions of the Southwest,” Springer said. “He followed up the quantitative analyses with innovative interpretations related to long-term management of groundwater in the Colorado River Basin.”
The collection of data and methods from this study are an important contribution to understanding the sustainability of the river. Better policy decisions can be made on river management, but the methods need be extended to the other subbasins of the Colorado River to quantify the base-flow for the entire basin. Once this knowledge is obtained, an even more sustainable solution for management practices can be established.
“Water in the West is a very contentious issue and this research analyzed groundwater policies to reveal the diversity and efficacy of groundwater governance surrounding the Colorado River,” Swanson said. “This research has brought together a lot of my interests and has done so in a way that addresses very important issues.”
As research moves forward, Swanson already has a second journal article forthcoming focused on the policy side of water management based on the results of this study.