T171 Historical Range of Variability in Landscapes

Tuesday, 2 November 2010
8:00 AM - 12:00 PM
Room 406

HOLOCENE AND HISTORIC VARIABILITY IN BEAVER DAMMING on SMALL STREAMS IN YELLOWSTONE: ASSESSING HUMAN AND NATURAL CONTROLS

Paper number: T171-4
Program: Technical Sessions
Time: 8:40 AM – 8:55 AM

Authors

Abstract

In northern Yellowstone (YNP), thriving beaver colonies on small streams in the 1920s were abandoned by the 1950s and remain essentially devoid of beaver activity today. The decline of beaver has been attributed to wolf extirpation, which allowed increased elk populations to over-browse willow and aspen, key food and dam-building resources for beaver. Historical loss of beaver dams has been inferred to have caused widespread channel incision into beaver pond sediments and alluvium, with lowering of water tables and an overall landscape transition from a “beaver-willow” to an “elk-grassland” state. This model is based on local historic changes, with little reference to longer-term background variability or larger-scale geomorphic and climatic controls on small streams. We used Holocene beaver pond sediments preserved in fluvial deposits of northern YNP and Grand Teton NP to investigate the long-term effects of beaver and climate change on riparian landscapes. Beaver pond sediments are preserved intermittently along ~29% of the total stream network length, in reaches with lower stream power; only 8% of the total shows evidence for channel incision following historic beaver dam abandonment. 14C dating shows that incision in some reaches predates historic abandonment. Beaver pond sediments are mostly < 2 m thick, except in glacial scour depressions undergoing long-term filling.

Most 14C dates of beaver pond sediments fall after ~4000 cal yr BP, in part due to greater preservation and exposure, but also consistent with a cooler and wetter Neoglacial climate that likely raised base flows. Within this time, however, centuries of minimal pond sedimentation occurred, with local channel incision and deposition of coarse flood gravels that disrupted beaver wetlands. These changes correspond with regional episodes of drought and severe wildfires, e.g. the Medieval Climatic Anomaly ~750-950 cal yr BP, when reduced base flows may also have impacted beaver habitat. Similarly, some small spring-fed tributaries that supported beaver in the 1920s have run dry in recent droughts. The combined geologic and historic record indicates that elk browsing alone cannot fully explain beaver variability, and that future severe droughts may have major impacts on beaver, especially in small catchments with limited water availability.