THE IMPACTS OF BEAVERS, FIRES, AND LATE HOLOCENE CLIMATIC VARIABILITY ON RIPARIAN ECOSYSTEMS IN THE GREATER YELLOWSTONE ECOSYSTEM

Small first to fourth order streams in lower elevations of the Greater Yellowstone Ecosystem sustain riparian ecosystems in an otherwise dry landscape. Beaver are an important component of these ecosystems, and can increase riparian habitat through dams that decrease stream velocity, increase the wetter perimeter of the channel, raise floodplain water tables, and force overbank flow. The ability of beaver to occupy small streams, however, is dependent upon perennial discharge. Late Holocene climatic variability has caused severe multidecadal droughts that have decreased discharge, for example during the Medieval Climatic Anomaly (MCA; ~900-1300 CE). In northern Yellowstone, severe droughts during the MCA promoted severe forest fires and fire-related debris flows on alluvial fans. Small streams experienced large postfire floods that likely caused channel incision, and that deposited coarse gravels in otherwise fine-grained floodplain sequences. During the MCA, a paucity of beaver pond sediments suggests that beaver damming is also dramatically reduced during this time. Decreased beaver activity may be the result of low flows and channel instability during drought. Average August discharge at USGS stream gauge locations was estimated for the late Holocene using the Palmer Drought Severity Index as a proxy. Stream discharge was reduced 25-40% during the most severe 2-yr drought in the MCA. Reconstructions were extended to ungauged basins using multiple regression analyses where basin area, mean basin elevation, basin area above 2400 m, and August insolation are significant predictors of discharge. The resulting estimates indicate that during the most severe 2-yr drought in the MCA, 60% of stream reaches with documented late Holocene beaver activity became ephemeral. This result is consistent with the observation of ephemeral flow during the severe 2000s CE drought on stream reaches where beaver were present in the relatively wet 1920s CE. In the coming century, most models predict that the greater Yellowstone Ecosystem will likely experience more frequent and more severe drought. Beaver activity on smaller streams will likely be markedly reduced, exacerbating impacts to riparian habitat.