Status and trends of springs at Hovenweep National Monument, 1999–2021

Weissinger, Rebecca

doi:10.36967/2294373

Water is a scarce, but vital, resource at Hovenweep National Monument (NM). The National Park Service has prioritized long-term monitoring of water resources at the monument through a variety of programs and indicators since 1999. The purpose of this report is to evaluate water-quantity and water-quality data collected at long-term monitoring sites in Hovenweep NM from 1999 to 2021 for trends over time, and to summarize site-characterization data for currently monitored locations. Data are available for three active monitoring stations—Square Tower Spring, Hackberry Pool, and Goodman Point Spring—and three discontinued monitoring stations—Cajon Pool, Upper Hackberry Spring (Spring #4), and Horseshoe Spring. Water resources at Hovenweep NM have exhibited many trends over the past two decades. Pool levels at both Square Tower Spring and Hackberry Pool went up, with a step increase shown in 2016 (after no sampling in 2014–2015). In contrast, flow decreased by more than half at Goodman Point Spring and ceased at Upper Hackberry Spring (Spring #4). Flow at Horseshoe Spring also decreased, although the period of record at this site was very short. Both Upper Hackberry Spring (Spring #4) and Horseshoe Spring were removed from the monitoring rotation because flows were increasingly absent or too low to measure. Pool-level dynamics and water chemistry at Cajon Pool are characteristic of precipitation-derived water rather than groundwater; this site was removed from the monitoring rotation because it was not a good indicator of groundwater conditions. The water chemistry in both Square Tower Spring and Hackberry Pool changed significantly, but in opposite directions. Square Tower Spring had increasing total dissolved solids (TDS) and specific conductance, with all major ions showing increases over time. Hackberry Pool had decreases in TDS and most major ions through 2010, when water-chemistry lab analyses were suspended due to logistical constraints. Specific conductance continued to decrease at Hackberry Pool through 2021. The reasons for these changes are unknown. Increases in major ions at Square Tower Spring are unlikely to be related to oil and gas development in the region. Decreases in major ions at Hackberry Pool may reflect an increase in the amount of water that is entering the pool from rain and snowmelt rather than groundwater at this site. Water-quality parameters at Goodman Point Spring have remained stable. While the causes of trends in water chemistry at these sites are likely natural, the small quantities of water and harsh water-quality conditions make these sites poor habitat for aquatic life. Groundwater is naturally low in dissolved oxygen—but the dissolved-oxygen concentrations in the two pools are persistently low, sometimes falling to near zero in the summer. Water temperatures also warm in conjunction with air temperatures, with temperatures as warm as 18°C in the pools and 19°C in Goodman Point Spring. Traditionally, Puebloan cultures would have managed these small water resources to promote infiltration and retention of precipitation and to remove sediment and debris. Decreases in flow and increasing vegetation stress are likely tied to increasing air temperatures and long-term drought conditions across the region. The namesake hackberry trees at Hackberry Pool and nearby sites have shown increasing drought stress. Regionally, the underlying Dakota aquifer that feeds springs at the monument was relatively stable from 2008 to 2017, with decreasing levels from 2018 to 2021. Overall, the most concerning trends observed at springs in Hovenweep NM are likely due to climate change. Increasing air temperature threatens springflow and spring-associated vegetation through increased evapotranspiration. While pool levels have increased, recent decreases in the regional Dakota aquifer raise concerns about the stability of pools in the future. Managers could consider the potential benefits and risks of: Prioritizing protections and actions at Square Tower Spring, which has been the most resilient site at the monument in persistence of surface water and vegetation health during extreme drought. Implementing potential climate-adaptation actions that could improve groundwater infiltration upgradient of spring sites. This could include rehabilitating traditional structures to slow the flow of surface water. Implementing traditional management techniques that could increase dissolved-oxygen concentrations in the spring pools. Increasing invasive-plant removal efforts at Goodman Point Spring, which had the greatest number and cover of invasive-plant species of the three currently monitored locations. Developing interpretive stories about water-management techniques and the role of climate change in creating drought stress on iconic hackberry trees.

Status and trends of springs at Hovenweep National Monument, 1999–2021

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