Medieval-Period Droughts, Fires, Floods and Geomorphic Change In Interior Western USA Mountains

We employ radiocarbon dating of hillslope-alluvial systems in diverse mountain forests and rangelands to understand climate-influenced Holocene geomorphic changes. In high-elevation conifer forests of northern Yellowstone (nYNP), fire- and storm-induced slope erosion over Medieval time caused tributary alluvial fans to aggrade and prograde over mainstem floodplains, and a pulse of large fire-related debris flows is associated with the ca. 1150 AD megadrought. A gap in beaver occupation of small nYNP streams 1000-1200 AD is likely drought-related, and channel incision and coarse gravel deposition provide evidence for high-energy floods during this interval. Despite major slope sediment production, larger streams downcut within Medieval time, indicating excess stream power. Historic analogs suggest that extreme floods were generated by rapid snowmelt and rain-on-snow associated with anomalous spring-early summer warmth, but anomalously wet climate may also have produced heavy snowpacks and runoff. In central Idaho, large fire-related debris flows indicate severe fires ranging from lower-elevation ponderosa pine stands to higher mixed-conifer stands between 950-1150 AD, when ~25% of total fan aggradation over the last 2000 yr occurred. In the ensuing, effectively wetter Little Ice Age, fire-related sedimentation consisted of more frequent but small events, when greater moisture allowed more grass growth and surface-fire activity; this is also consistent with more frequent fire-related sedimentation in nearby rangelands. In ponderosa-mixed conifer forests of the Sacramento Mountains, southern New Mexico, rapid fire-related aggradation of fans and valley floors occurred during Medieval time, with a major pulse of debris flows during the “Great Pueblo Drought” of the late 1200s AD. Preceding unusually wet decades may have increased forest densities, making severe fires more likely. However, even greater fan aggradation occurred ca. 6000-4000 cal yr BP in both the Sacramentos and nYNP. Overall, the highly variable, drought-prone climate of Medieval time forced rapid geomorphic change, especially in forested mountain environments.