HYDROGEOMORPHIC SETTING, CHARACTERISTICS, AND RESPONSE TO STREAM INCISION OF MONTANE RIPARIAN MEADOWS IN THE CENTRAL GREAT BASIN—IMPLICATIONS FOR RESTORATION

Riparian wet meadow complexes in the mountains of the central Great Basin are scarce, ecologically important systems that are threatened by stream incision. An interdisciplinary group has investigated 1) the origin, characteristics, and controls on the evolution of these riparian meadows, and 2) the hydrologic and vegetative response to stream incision. Detailed data on groundwater (60-100 wells per site), surface water (gaging), geomorphology, stratigraphy (dozens of cores), geophysics (seismic and GPR), and vegetation have been collected in four meadow complexes with reconnaissance level data collected in dozens.

Meadows are present in groundwater discharge zones with high flow rates that are geologically controlled to maintain high groundwater tables. Many of the meadows are present upstream of side-valley alluvial fans. The interplay of faults, differential sediment inputs from down valley and side valley sources, and alluvial fan form and sediments create conditions necessary for riparian meadows. Hydrologic conditions to support meadows can also be created by constrictions in alluvial fill either by basin shape or underlying bedrock geometry. Meadows terminate downstream where unconfined, high hydraulic conductivity layers permit rapid draining and lowering of the groundwater table. Artesian conditions are common in meadows because of the complex stratigraphy with strongly contrasting hydraulic conductivities and high hydraulic gradients.

Many meadow complexes are undergoing stream incision because of current climatic conditions, basin sediment supplies, geomorphically and hydrologically sensitive conditions, and anthropogenic impacts. Entrenchment of meadows is enhanced in many places by groundwater sapping. Incision lowers the groundwater table, causes shifts in vegetation from wetter to drier species, may change groundwater flow paths, and, in extreme cases, may cause permanent loss of the meadow complex. The magnitude and type of response to incision varies with groundwater budget, hydraulic conductivity of sediments, and stratigraphy. Proper management of the wet meadow complexes, including stream restoration or stabilization, requires understanding the spatial and temporal complexities characteristic of these systems.