GEOMORPHIC AND HYDROGEOLOGICAL CONTROLS ON THE DISTRIBUTION OF WET MEADOWS IN THE CENTRAL GREAT BASIN

The Great Basin is an arid landscape dominated by dryland vegetation such as big sage and xeric grasses. Meadow complexes occur in mountain drainages and consist of discrete parcels of land up to several hectares in area that are characterized by high water tables and that primarily support willows, wetland grasses and sedges. Although these meadow complexes make up less than 1% of the landscape by area, they play a very important role ecologically in terms of the numbers of endemic and migratory species that rely on them. Because meadow complexes are both sparse and ecologically valuable, it is important for land managers to understand the natural controls on their distribution for proper land management.

Meadows exist only in locations where the groundwater table remains close to the surface throughout the summer. Water can be supplied to wet meadows from several sources occurring either singularly or in combination. Water sources may include: (1) bedrock-source groundwater springs that derive their recharge within the drainage basin, (2) bedrock-source regional scale groundwater springs that transfer water from one watershed to another, and (3) alluvial-fill-source water moving down valley through the valley-fill sediments.

Although groundwater discharge and high water tables are essential for meadows, groundwater discharge alone is insufficient to form and sustain a meadow. Meadows also require circumstances in the geologic, geomorphic, and sedimentologic architecture of the site that causes the down-valley flux of groundwater to be detained so that groundwater levels rise to the surface. Constrictions most commonly occur where bedrock valleys narrow and alluvium from tributary valleys accumulate in the axial valley. In many cases, these side valleys produce well-defined alluvial fans that prograde into or across the axial valley. This setting leads to a complex stratigraphic package, including fine-grained materials, that create the conditions necessary to detain groundwater and promote shallow groundwater tables.