Climatically Controlled Water Supply in the Bryce Canyon Region?

by Janae Wallace, Kathryn Ladig, and Hugh Hurlow

Southwestern Utah is a scenic and beautiful area that hosts several national and state parks with accompanying growing populations; the valleys that surround Bryce Canyon National Park are not unique in this scenario. Johns and Emery Valleys are home to about 360 permanent residents, but millions of tourists visit the area annually and that number is increasing. For example, approximately 1.7 million people visited the park in 2015 compared to 2.4 million people in 2022. Water supply and demand may control future development surrounding this geologic wonder, creating a challenging hurdle for local and state managers.

Map of Bryce Canyon study area. Highlighted is the many streams or canals within the study area.

Map of study area

In the Bryce Canyon region, water generally is provided by groundwater from wells; a seasonal increase in population will likely increase the consumption of this water supply. Valley-fill sediments host the principal groundwater aquifer; however, groundwater storage in this aquifer is relatively limited based on its thickness (less than 300 feet). Although land use and development in the study area is centered around tourism, agriculture remains an important land use in Johns and Emery Valleys. In 2021, over 7,700 acres of land were designated as irrigated, sub-irrigated, or non-irrigated agricultural use. Irrigation in the study area is from surface water sources typically, including the Tropic Ditch in Emery Valley and mountain streams and springs in Johns Valley. Potential future development and the threat of future drought in southwestern Utah prompted our study to develop a groundwater budget, or an accounting of incoming and outgoing water to and from the aquifer, for Johns and Emery Valleys.

We began this project by conducting a hydrogeologic study after which a water budget could be calculated for the valleys. To shed light on identifying a groundwater–surface-water relationship, we measured flow in streams and canals, including an irrigation diversion known as the Tropic Ditch, and measured water levels in wells to generate maps showing contours of water-level elevations used to help understand groundwater flow in the valley. In estimating recharge to the system, we assumed that the surface-water drainage boundary is a groundwater divide, which precludes groundwater inflow from adjacent hydrologic basins. Therefore, the only primary input to the system is precipitation. Water can leave the system by three primary means: evapotranspiration, discharge to the Tropic Ditch, and discharge from the East Fork Sevier River.

We used a soil-water balance (SWB) model to understand the interaction between surface water and the sediments of the valley-fill aquifer. The spatial data used for the model included Daymet climate data, a digital elevation model for calculating water flow direction, a descriptive soils layer, and land cover data. The SWB model shows precipitation averaged approximately 383,000 acre-feet/year and evapotranspiration averaged 372,000 acre-feet/year over the entire study area. The model also indicated an average recharge to the aquifer of 9,400 acre-feet/year and average net loss of 11,000 acre-feet/year.

To validate the SWB model, we used data derived from well water-level measurements and stream and spring seepage measurements of the East Fork Sevier River and tributaries. We measured water levels in about 30 wells during the autumn and spring of 2018 through 2022. Our seasonal and annual data show water levels in most valley-fill wells fluctuate depending on winter precipitation. These wells had relatively short-term increases and declines depending on snowfall amounts, indicating storage in the aquifer is limited. The potentiometric surface—watertable level in the subsurface—in the valley-fill aquifer generally increases (rises) in the spring and declines in the fall. Water levels in wells completed in bedrock aquifers were less variable than wells completed in the valley-fill aquifer, with some water levels declining and others rising during different seasons and years, weather independent. Because water level in valley-fill wells increases after heavier winter snowpack, we conclude that the valley-fill aquifer is more sensitive to precipitation from surface water runoff and direct infiltration than the bedrock aquifers. Groundwater pumping also likely contributes to water-level fluctuations in the valley-fill aquifer, particularly along the more densely developed Highway 12 corridor.

Seepage runs—measuring streamflow on multiple sections of a watercourse in as short a time span as possible—allowed us to identify gaining and losing reaches of streams and canals, helping us better understand critical zone processes and the study area’s water budget. (The “critical zone” is the physical nexus among shallow groundwater, surface water, the atmosphere, and vegetation.) During 2018, 2019, and 2020, we performed a total of four seepage runs on the East Fork Sevier River and Tropic Ditch in the lower reaches of Emery and southern Johns Valleys. We expanded the study area to include all of Johns Valley to the north into Black Canyon and performed seepage runs in October 2021 and May 2022. Discharge measurements from seepage runs show that the East Fork Sevier River has both gaining and losing reaches that vary in position and length depending on the time of year and groundwater conditions in the adjacent valley-fill aquifer. Based on our observations, the valley-fill aquifer in Johns and Emery Valleys receives recharge from surface water (is net gaining) when surface water is actively flowing through the valleys and the East Fork Sevier River is not fully diverted to the Tropic Ditch. This dynamic transitions to net gaining to surface water at the north end of Johns Valley where the water table intersects the land surface and perennial wetlands are supported.

Block diagrams of a gaining stream, where the water table is high and flowing towards the stream, and a losing stream with a low water table flowing away from the stream.

Schematic diagram of A) a gaining stream and B) a losing stream.

Water-level data for a valley-fill aquifer well north of the Hwy 12 corridor showing slightly increasing water levels during spring snowmelt in 2019 and 2021. For the 2022–2023 water year, water levels steadily increased in direct response to the heavy winter snowpack and subsequent snowmelt, rising 14 feet in elevation. The gray bars represent March 1 through June 1 of each year.

Water-level data for a valley-fill aquifer well north of the Hwy 12 corridor showing slightly increasing water levels during spring snowmelt in 2019 and 2021. For the 2022–2023 water year, water levels steadily increased in direct response to the heavy winter snowpack and subsequent snowmelt, rising 14 feet in elevation. The gray bars represent March 1 through June 1 of each year.

Our study shows the valley-fill aquifer in Johns and Emery Valleys is recharged by precipitation and surface water, responding readily to fluctuations in climate. Wetter than average water years result in increased groundwater levels, whereas drier than average years result in decreased water levels; there is little storage in the aquifer to attenuate climatic fluctuations. In recent times, all water flowing beyond the Tropic Ditch diversion into Emery Valley has been lost to groundwater, highlighting the aquifer’s reliance on the East Fork Sevier River and its tributaries. We discovered substantial groundwater discharge in northern Johns Valley that supports an expansive wetlands system and stream flow in the East Fork Sevier River. Some of this groundwater likely recharges in the mountains in the northern part of the valley, far from currently proposed development, but some is derived from the valley-fill aquifer in the Emery/southern Johns Valley area. Extensive groundwater development in the area would cause the potentiometric surface in the aquifer there to decline, reducing the hydraulic gradient to the north and thereby capturing more flow from the East Fork Sevier River and some of the groundwater discharge in northern Johns Valley. Reduced groundwater discharge would potentially affect the groundwater-dependent ecosystem represented by the wetlands and decrease streamflow out of the valley. Reduced flow out of Johns Valley could impact water quality and supply issues in the surrounding region. For example, the Sevier River drainage basin suffered the greatest reduction in stream flow and reservoir storage in Utah during the 2021–2022 extreme drought (data source: USDA).

View to the south of an extensive perennial wetland area (~100 acres) that exists between an unnamed spring and the East Fork Sevier River (left of the photo, not shown).

View to the south of an extensive perennial wetland area (~100 acres) that exists between an unnamed spring and the East Fork Sevier River (left of the photo, not shown).

In summary, the valley-fill aquifer is sensitive to precipitation events—no snow, no water. Due to the close link between groundwater and surface water in Emery and Johns Valleys and limited groundwater storage in the valley-fill aquifer, lowering the water table may impact stream flow in an already vulnerable system. Scant long-term data indicate groundwater levels have historically fluctuated around a steady average, but extended drought could easily alter this balanced pattern and result in decreased water availability. The relatively recent period of drought has increased the percentage of total recharge to the aquifer that comes from the East Fork Sevier River, which depends on how Tropic Reservoir and the Tropic Ditch are managed. Because of the potential increase in growth from tourism-related development, an increased demand for drinking water warrants continuous monitoring that will assist land-use planning and resource management to maintain local water resources.

Janae Wallace

is a senior scientist who has been employed with the UGS in the Groundwater & Wetlands Program since 1996. She received a B.S. in geology from University of Utah and M.S. in geology from Northern Arizona University. Her principal duties include groundwater-quality  projects that focus on valley-fill aquifers, elevated nitrate concentrations in rural valleys, septic-tank density recommendation maps, environmental tracer analysis, pesticide sensitivity and vulnerability maps, watershed studies, and water well-cuttings analysis.

Kathryn Ladig

joined the UGS Groundwater & Wetlands Program in 2021. She earned a B.A. in geology and environmental studies from Gustavus Adolphus College and an M.S. in earth science from the University of Maine. Kathryn has studied geology throughout the globe and was employed previously by the National Park Service. Her passions lie in tracking the impacts of climatic variability through both proxy and direct observation.

Hugh Hurlow

joined the UGS in 1995 and is a hydrogeologist and the Program Manager of the Groundwater & Wetlands Program. He has a Ph.D. from the University of Washington, an M.S. from the University of Wyoming, and an Sc.B. from Brown University, all in geological sciences. His current focus is studying the hydrologic effects of large-scale environmental restoration of sagesteppe ecosystems.