BEAVERS, FLUVIAL GEOMORPHOLOGY, AND CLIMATE: HOLOCENE AND HISTORIC BEAVER IMPACTS ON STREAMS IN YELLOWSTONE NATIONAL PARK, WYOMING

Beaver damming has been considered by some as a viable mechanism for major aggradation of mountain stream valleys, but this hypothesis has not been tested with stratigraphic and geochronologic data. We use fluvial stratigraphy, ¹⁴C geochronology, and geomorphology to document the long-term impact of beavers on 1st- to 4th-order streams in the semiarid-subhumid Northern Range of Yellowstone National Park. Beaver pond deposits are generally less than 2 m thick and are preserved on all streams in the study area, but only in limited reaches. Maximum channel gradient at dam sites decreases downstream with increasing basin area in a power-law threshold relationship that indicates strong control by stream power on beaver damming and pond sediment preservation. The limited thickness and patchy distribution of pond deposits indicates that the overall potential of beavers to force aggradation of Northern Range stream valleys is small. The greatest amounts of beaver-related aggradation occur on low-gradient reaches, often in glacial scour depressions. Dating shows that relict beaver dams on low-gradient reaches can influence local channel morphology for as much as 1100 years. Thirty-eight ¹⁴C ages on beaver pond sediment fall primarily within the last 4000 yr, but gaps in beaver occupation from 2200-1800 and 750-950 cal yr BP correspond with episodes of severe and prolonged drought. This suggests that climate can affect beaver occupation through impacts on streamflows and potentially riparian vegetation. In the past century, northern Yellowstone has trended toward a warmer, drier climate, and some small streams that were perennial and beaver-dammed in the 1920s now run dry in the summer. The paucity of aspen and low summer streamflows suggest that the Northern Range is not presently conducive to widespread beaver damming, as in the early 20th century. The ensuing loss of beaver has been cited as a major cause of channel incision and overall riparian degradation in the Northern Range. However, historic channel incision in the study area is local in nature, less than ~2 m, and related to factors including beaver dam abandonment, stream capture, and fire-related floods. Incision on some reaches began before 1450 cal yr BP, and some earlier Holocene incision episodes were followed by channel filling. Channel incision thus cannot be solely attributed to beaver dam abandonment.