HR: 1340h
AN: H13E-1360    [Abstracts]
TI: Holocene Beaver Effects on Small Streams in Yellowstone and Implications for Stream Restoration
AU: * Persico, L P
EM: lyman@unm.edu
AF: University of New Mexico, Northrop Hall, Albuquerque, 87131
AU: Meyer, G
EM: gmeyer@unm.edu
AF: University of New Mexico, Northrop Hall, Albuquerque, 87131
AB: It has been asserted that beaver ( Castor canadensis) damming has sustained long-term aggradation and exerted a dominant control on the morphology of small streams over much of North America. However, data on the temporal and spatial dimensions of beaver influence are extremely limited. Using beaver pond deposits and berms (abandoned dams), we document geomorphic effects of beavers on first- to fourth-order streams in semiarid-subhumid northern Yellowstone National Park, USA. Beavers were ubiquitous in the early 20th century, but are currently rare. Some formerly dammed streams have become ephemeral in recent droughts, suggesting that climate may be a significant factor controlling beaver occupation. Radiocarbon dating of wood preserved in pond deposits and berms shows notable periods of beaver activity 3655-3855, 1555-955, and 455-150 cal yr BP, but a distinct lack of activity 950-700 cal yr BP during the Medieval Climatic Anomaly, a time of severe multidecadal droughts in Yellowstone and the western USA. The spatial scale over which beavers incurred significant aggradation is controlled largely by geomorphic settings conducive to damming. Low-gradient (0.07 to 0.001) reaches with contributing areas of 4 to 70 km² are typical for dam sites. Reaches with downstream valley constrictions are most susceptible to aggradation. Only a small fraction of the total stream length in the study area has experienced significant aggradation attributable to beaver damming, as shown by accumulations of sand and finer sediment of up to 3 m, thicker than typical overbank sediments. These sediments locally contain evidence of ponding in laminations, gleying, and high organic content. Many reaches show no evidence of any net aggradation since deglaciation. Many beaver-aggraded reaches are now incised, typically 1.5-2 m and up to 3 m. Some reaches have early Holocene (ca. 10150-8000 cal yr BP) terraces with treads ~2 m above current bankfull level, underlain by both gravelly and fine-grained fill deposits showing little evidence of ponding. Overall, these observations suggest that very high beaver populations of the early 20^th century were an anomaly and that their presence in the Holocene was intermittent, with only local major impact. This counters the assumption that most stream reaches in this and similar systems have been degraded largely because of beaver abandonment.
DE: 1815 Erosion
DE: 1824 Geomorphology: general (1625)
SC: Hydrology [H]
MN: Fall Meeting 2005