HYDROLOGY OF THE RHYOLITE PLATEAU, YELLOWSTONE NATIONAL PARK

The Rhyolite Plateau spans 1/4 of Yellowstone National Park (1900 km²) and its topographic position makes it an important headwater region of the park. The Plateau consists of young (72-484 ka) Quaternary rhyolite flows with broad upland surfaces (2500-2750 m elevation) and steep margins with 100-500 m relief. We hypothesize that the hydrology of the plateau is heavily influenced by surficial materials and the rhyolite flow morphology and structure and that the hydrology evolves through time.

Perennial streams on the Rhyolite Plateau are rare, even in basins with drainage areas >10 km². Most streams are intermittent, and even during the snowmelt season (June) many of the channels contain only discontinuous flow as meltwater readily seeps into the coarse (sand- to boulder-sized) channel sediment. Small lakes and springs occur on the plateau predominantly where bedrock flow structures dam or redirect surface runoff and subsurface flow. However, perennial springs are common around the margins of the flows, indicating that most water infiltrates into deep fractures where it is transported to the edge of the flow. We hypothesize that the early-season meltwater pulse may be delayed or extended at these flow-margin springs due to long flow paths (slope lengths are ~3-15 km) and low permeability within deep bedrock. Support for this hypothesis comes from preliminary observations of some springs having more discharge in late summer, when local stream and river discharge was lower. These features may therefore be an important source of late-season surface water critical for wildlife that depend on perennial water sources.

Rhyolite Plateau hydrology appears to evolve through time, with younger flows having greater flow-margin spring discharge and older flows having more perennial streams. We hypothesize that runoff is enhanced by the accumulation of fine aeolian and glacial sediments over the fractured bedrock, and deep fracture flow is inhibited by bedrock weathering, hydrothermal alteration and plugging of fractures by translocation of surficial materials. Streams on older flows have incised more (relative to basin size and relief) and have access to more subsurface flow; therefore, these perennial streams may exhibit a more efficient drainage of the early-season meltwater pulse as compared with the flow-margin springs.