Wildfire and the patterns of floodplain large wood on the Merced River, Yosemite National Park, California, USA

We quantified floodplain large wood load (m3 wood/ha) and spatial distribution on the Upper Merced River in Yosemite National Park, California, USA. The upstream portion of the study area includes a recently burned section of the Merced River corridor and the downstream portion contains floodplain with undisturbed forest, facilitating investigation of the effects of wildfire on floodplain large wood. We used measurements of wood load and spatial distribution to test hypotheses regarding floodplain wood dynamics in the channel versus the floodplain and in burned versus unburned portions of the study area. The median wood load on the Merced River floodplain, as measured along numerous transects, is 259 m3/ha overall, with non-significant differences between burned (median 196 m3/ha) and unburned (median 277 m3/ha) portions of the floodplain. We found that jams can occur across the entire width of the floodplain. Burned wood pieces are present throughout the study area in the channel but are largely absent from unburned portions of the floodplain, despite the occurrence of overbank flows since the wildfire. A greater proportion of large wood is within logjams in burned portions of the floodplain. We infer that wood recruited to the channel via bank erosion moves readily downstream within the channel, whereas wood moving from the channel onto the floodplain concentrates near the margin of the main channel or within secondary channels and depressions on the floodplain, leading to the formation of long, narrow logjams.