Space- and time-scale variability of fens and other groundwater-dependent ecosystems in the Northern Prairies

Background/Question/Methods

The northern US prairies lie in a transitional zone between humid and semi-arid regions where evapotranspiration exceeds precipitation. Recharge to groundwater is limited in most places, indicating that areas of focused natural discharge require large capture zones to provide sufficient groundwater flow. The discharge areas create groundwater-dependent ecosystems (GDEs) that are small and sparsely scattered across the landscape. These two characteristics increase the conservation importance of prairie GDEs.

To understand how to protect and restore GDEs, essential information includes quantification of the water budget and determination of the groundwater capture zone. The latter is often different from the surficial watershed because of subsurface hydrogeology. Various data and analytical methods were used to characterize GDEs: stratigraphic records and fence diagrams, composition of the discharged water, geophysical surveys, measurement of water-budget components, and flow modeling. Combination of this information with topographical and hydrogeological analysis at various scales revealed the occurrence of GDEs and their associated groundwater capture zones.

Results/Conclusions

Northern prairie GDEs form a broad spatial and temporal continuum. The brackish wetlands of North Dakota’s Red River Valley are related to the geohydrology of the sub-continental-scale Williston Basin. In contrast, recent monitoring shows that ephemeral fens occur along the margins of the Red River Valley where shallow groundwater discharges only sparingly in the spring. At Juneberry Fen, northwest Minnesota, groundwater alkalinity, a characteristic of calcareous fens, developed through the "push-and-pull" of evapotranspiration and precipitation in the phreatic zone, which creates groundwater circulation in the absence of topographic head. Temporally, active springs and continuous flow through hydrodynamic blowouts, 10,000 year-old features related to glacial Lake Agassiz, demonstrates that groundwater discharge has occurred for millennia and created a climate-change refuge for relict species. In other cases, saline seeps of Montana and western North Dakota are nascent GDEs that began to develop several decades ago through a concomitant increase of recharge with cultivation. Both water budget and water quality in the region's GDEs have been disrupted, often irreversibly, by changes in drainage, capture zone size, and land use. Regardless of origin, characterizing the source, path, and  balance of water is an essential first step in restoring and protecting prairie GDEs.