Influence of geologic setting on slope wetland hydrodynamics

Slope wetlands exist where topographic or stratigraphic conditions allow ground water to intersect the surface, creating a zone of perennial or near-perennial moisture. The condition and resiliency of slope wetlands are, therefore, controlled by their hydrodynamics (i.e., the movement and action of water in the wetland) and recharge mechanisms. Understanding the mechanisms by which water flows into and through the wetland is essential for accurate assessment of potential indirect impacts and for the development of management measures geared toward preserving slope wetland functions. We investigated the physical and biological properties of twenty slope wetlands in southern California, USA to gain a better understanding of how their geologic setting influenced water movement through the wetlands and of the plant community composition in the wetland. Principle Components Analysis (PCA) separated the slope wetlands into three distinct groups based on geologic setting: those located in bedrock landslides, those associated with faults, and those associated with alluvial/colluvial deposits. Ground-water monitoring and hydrogeologic analysis showed that wetlands in alluvial/colluvial deposits respond quickly to precipitation, and subsurface water levels stay near the ground-surface elevation for extended periods of time. In contrast, subsurface water levels in bedrock landslide slope wetlands respond more slowly to precipitation, show greater variation over time, and ultimately decline more quickly after the cessation of recharge events. These observations (along with analysis of ground-water chemistry) indicate that wetlands in alluvial/colluvial deposits are likely supported by large, relatively stable volumes of ground water. In contrast, wetlands located in bedrock landslides are likely recharged from relatively localized ground-water sources with smaller storage volumes and greater interannual variability. Wetlands located along faults have an intermediate level of variation in moisture regime, indicating that their association with a fault may be providing a conduit for water delivery to the wetland. Plant species diversity did not differ between subclasses, although wetlands in alluvial/colluvial deposits supported slightly greater proportions of alkaline plant species. Understanding the different recharge characteristics of each subclass will allow for more informed decision-making regarding protection and management of slope wetlands.