The Great Basin is characterized by a series of north-northwest trending mountain ranges with broad central valleys where drainage is almost completely internal. More than 200 separate hydrologic systems can be delineated and about half of these are characterized by closed drainages. Mountain ranges can reach elevations in excess of 3500 m and, because of the large elevation gradients, precipitation and temperature varies significantly from the upper elevations to the mouths of the watersheds. Precipitation approaches 55 cm at upper elevations but decreases to as little as 20 cm in the central valleys. Most precipitation falls during the winter as snow, and peak runoff and most flood events occur during snowmelt in late May to early June. Convective summer storms occasionally result in flash floods. The upland watersheds are typically characterized by narrow valleys, and the stream systems are high gradient, coarse-grained, and often highly incised. While low flows in these stream systems range from about 0.015 to 0.063 m3/s, high flows can range from 0.214 to 0.683 m3/s. Most of the change in stream channel pattern and form occurs during high magnitude, low frequency flows. Many of the stream systems are ephemeral, and most of the perennial streams are diverted to agriculture lands where they flow out of upland watersheds and into the central valleys. Vegetation changes significantly along the elevation gradient from the central valleys to the mountain peaks. Riparian vegetation consists of narrow stringers of quaking aspen (Populus tremuloids), river birch (Betula occidentalis), cottonwoods (P. angustifolia and P. trichocarpa), willows (Salix spp.), and wild rose (Rosa woodsii ). Meadow communities occur in valley segments with groundwater control and elevated water tables, and are dominated by grasses, sedges (Carex spp.), and rushes (Juncus spp.).
Riparian areas are important components of all landscapes, but in the semi-arid Great Basin they constitute an especially vital resource. Although riparian areas comprise less than 1% of the Great Basin, they supply many critical ecosystem services including water for culinary and agricultural uses, forage and browse for native herbivores and livestock, and recreational opportunities. They provide habitat for a variety of organisms such as butterflies and neotropical migrant birds, and support a relatively high number of endemic species, including the Lahontan cutthroat trout which is listed as threatened under the U.S. Endangered Species Act.
In the central Great Basin riparian areas exhibit widespread degradation and much of this degradation is related to stream incision or down cutting. The processes currently influencing stream incision and riparian ecosystem degradation are strongly influenced by past climates. Paleoecological records indicate that a major drought occurred in the Region from approximately 2500 to 1300 YBP. During this drought, most of the available sediments were stripped from the hillslopes and deposited on the valley floors and side-valley alluvial fans. As a consequence of this hillslope erosion, streams in the central Great Basin are currently sediment limited and exhibit a natural tendency to incise. In fact, the geomorphic data indicate that over the past 2000 years, the dominant response of the streams to disturbance has been incision. Since settlement of the region in 1860, the rate and magnitude of steam incision in central Great Basin watersheds have been significantly increased by anthropogenic disturbances like roads in the valley bottoms. Analyses of central Great Basin watersheds indicates that the tendency of a particular stream to incise is strongly influenced by the sensitivity of the watershed to both natural and anthropogenic disturbance. The tendency of a stream to incise can be predicted by watershed characteristics such as geology, size and morphology, and valley segment attributes like gradient, width and substrate size. This type of information is being used to develop process based approaches for restoring and maintaining these valuable ecosystems.