Structural organization of process zones in upland watersheds of central Nevada and its influence on basin connectivity, dynamics, and wet meadow complexes

摘要

The drainage network within upland watersheds in central Nevada can be subdivided into distinct zones each dominated by a unique set of processes on the basis of valley form, the geological materials that comprise the valley floor, and the presence or absence of surficial channels. On hillslopes, the type and structure (frequen cy, length, and spatial arrangement) of these process zones is related to the lithology and weathering char acteristics of the underlying bedrock. Process zones dominated by sediment accumulation, storage, and groundwater recharge are associated with less resistant rocks that weather into abundant but relatively small particles. Sediment transport and runoff-dominated zones are associated with resistant, sparsely frac tured rocks that produce limited but larger clasts. The type and structure of process zones along axial valleys depend on the characteristics of the process zones on the hillslopes. Numerous sediment storage-dominated reaches leads to a relatively high number of unincised fans located at the mouth of tributaries along the axial valleys and to frequent and lengthy unincised valley segments, both of which disconnect large sections of the drainage basin from channelized flows. In contrast, a relatively high density of transport-dominated process zones leads downstream to the incision of side-valley fans and axial valley deposits as well as a high degree of basin connectivity (defined by the integration of surficial channels). Connectivity also is related to the lithol ogy of the underlying bedrock, with higher degrees of connectivity being associated with volcanic rocks that presumably yield high rates of runoff. Lower levels of connectivity are associated most frequently with exten sively fractured, locally permeable sedimentary and metamorphic rock assemblages. Thus, basins underlain by volcanic rocks appear to be more sensitive to incision and produce more dynamic channels in terms of the rate of channel/valley modification than those underlain by other lithologies. Considerable attention has been devoted in recent years to the management of wet meadow ecosystems that serve as important riparian habitats within upland basins of central Nevada. The data presented here show that, within basins characterized by a high degree of connectivity, areas of wet meadow are minimal. Where they do exist in these basins, they tend to be severely degraded by incision or gully erosion and will be difficult to manage given the dynamic nature of the axial channel processes. Wet meadows within ba sins that exhibit a low degree of connectivity and high sediment storage-to-transport ratios on hillslopes will likely be more responsive to management activities because of the reduced threats of channel incision and, presumably, a larger supply of groundwater flow to the meadows created by an extensive network of re charge sites. Importantly, human activities that lead to an increase in basin connectivity can negatively im pact downstream meadows through a decrease in groundwater recharge and an increase in stream dynamics, in spite of the fact that these activities may be physically separated from the wet meadow areas.