HYDROLOGIC, GEOMORPHIC, AND VEGETATION RESPONSES OF A SPRING-FED RIPARIAN AREA IN GRAND CANYON NATIONAL-PARK TO A HUMAN CAUSED FIRE

Wildfire is a common phenomenon in desert wetlands, where productivity and fuel accumulation is often orders of magnitude greater than that in the adjacent uplands. A human-caused fire in a groundwater-fed riparian ecosystem in Grand Canyon National Park provided a unique opportunity to measure springs ecosystem responses and inform mitigation. We monitored West Fork of Cottonwood Creek (WFCC) in the Cottonwood Springs drainage to determine seasonal and post-fire responses in 1) hydrologic function, 2) channel morphology, and 3) vegetation regrowth. Debris flow potential was assessed by channel morphology monitoring, stream competency calculation, and historic and locally observed precipitation values. We characterized hydrology, channel morphology, and vegetation cover during pre- (springtime) and post-monsoon (late summer) precipitation periods. Post-monsoon watershed analysis revealed storm driven erosion and impacts on hydrologic function and vegetation regrowth in the burn area. We hypothesized that debris flow potential was low, and that monsoon-driven erosion would not have significant impacts on channel morphology or vegetation regrowth. Increased ash and sediment load caused the spring-fed reaches of the channel to remain braided throughout the burn area, rather than incising where it previously had been previously. Surface water volume decreased overall due to evapotranspiration from rapidly resprouting riparian species and increased soil temperature. Vegetation cover in the burn area increased due, in part, to increased total carbon and nitrogen; however, species distribution differed compared to pre-fire conditions due to differential plant life history strategies, including stand replacement due to resprouting versus seedling establishment. Fremont cottonwood (Populus fremontii) demography changed because mature trees sustained elevated mortality and potential seedlings were excluded by extensive understory species resprouting. Thus, while desert springs wildfire is a common phenomenon, it alters geomorphology and ecological functionality, creating a new ecosystem developmental trajectory and contributing to the highly individualistic nature of springs ecosystems. The methods we used in this study can be applied to similar systems in arid regions to better predict riparian ecosystem responses to future wildfire, informing management and restoration decisions of these keystone habitats in arid regions.