We use beaver-pond deposits and geomorphic characteristics of small streams to assess long-term effects of beavers and climate change on Holocene fluvial activity in northern Yellowstone National Park. Although beaver damming has been considered a viable mechanism for major aggradation of mountain stream valleys, this has not been previously tested with stratigraphic and geochronologic data. Thirty-nine radiocarbon ages on beaver-pond deposits fall primarily within the last 4000 yr, but gaps in dated beaver occupation from ~ 2200–1800 and 950–750 cal yr BP correspond with severe droughts that likely caused low to ephemeral discharges in smaller streams, as in modern severe drought. Maximum channel gradient for reaches with Holocene beaver-pond deposits decreases with increasing basin area, implying that stream power limits beaver damming and pond sediment preservation. In northern Yellowstone, the patchy distribution and cumulative thickness of mostly < 2 m of beaver-pond deposits indicate that net aggradation forced by beaver damming is small, but beaver-enhanced aggradation in some glacial scour depressions is greater. Although 20th-century beaver loss and dam abandonment caused significant local channel incision, most downcutting along alluvial reaches of the study streams is unrelated to beaver dam abandonment or predates historic beaver extirpation.

The Sino-Mongolian beaver Castor fiber birulai lives in the Ulungur watershed in China and Mongolia, an area little known to the people outside this region. We recorded the number of families, adults, subadults and juveniles at each of three beaver lodges in the Bulgan Beaver Nature Reserve in 2003, 2006 and 2007. Along the whole Ulungur watershed in China in each of the 3 years we conducted surveys for the beaver and estimated the total population based on the area of food caches. We recorded 135, 167 and 145 Sino-Mongolian beaver colonies and estimated a population 472–599, 543–700 and 508–645 beavers in 2003, 2006 and 2007, respectively. From 1989 to 2007 the number of human households in the Bulgan Nature Reserve increased by 112% and the population by 71%. Consequently human activities in the river valley, including collection of wood for fuel, increased. We also surveyed the site of a relocation of C. f. birulai from the Bulgan River to the Ertix River in 1992 but found no sign of living beavers. The population of the Sino-Mongolian beaver in China is small and restricted geographically. Threats, such as habitat modification and deterioration and competition with people for wood, continue. We recommend that the Bulgan Beaver Nature Reserve be expanded by a transfrontier agreement with the appropriate authorities in Mongolia, and that a plan for sustainable wetland management and restoration is required.