Hydrologic changes and processes underlying recent wetland loss in Yellowstone National Park

Wetlands are some of the most biologically productive yet vulnerable ecosystems on Earth. They provide essential habitat for various biota and act as landscape indicators by reflecting the status of catchment-scale processes. The drying and shrinking of wetlands during the past four decades in Yellowstone National Park's Northern Range has recently incited concern among National Park managers and the public at large. Investigation of wetland hydrologic regime is a critical step in building an understanding of these changing ecosystems. Our research has the following objectives: (1) Classify wetlands according to their particular hydrologic function, including climatic and geomorphic processes supporting them, (2) Determine the patterns and magnitude of water level declines that occurred during the late 20th and early 21st centuries and assess whether these fall within the natural range of variation, (3) More closely examine a focal site that has experienced dramatically reduced water levels to gain a more refined understanding of wetland processes. In 2009 we established a monitoring network of 24 wetlands within the Northern Range. Each wetland was instrumented with 4 to 6 shallow groundwater well and piezometer nests. Well data was manually collected from each site at one to two week intervals in summers 2009 and 2010. Data analyses indicate that the study sites represent locations of ground water discharge, recharge, and flow-through, as well as sites perched above the regional water table. We classified wetlands into 7 groups using a hydrograph shape-magnitude framework previously used in stream systems. Climatic data reveal that hydrologic conditions occurring in the recent past are within the range of historic variation, but that we are in a drier than average period. Aerial photographs and wetland soil delineation both reveal greater wetland extent in the past 50 years, and these conditions are linked to the environmental setting of each wetland. Wetland vegetation is shown to inhabit zones of specific water table fluctuation patterns, and thus can be used to infer subsurface hydrology in the absence of hydrologic data. In continuing analyses we will synthesize these wetland variables into a comprehensive view of wetland prevalence in Yellowstone's Northern Range, and consider this phenomenon in the context of global climate change.