BEAVER DAMMING AND DISCONTINUITY ALONG STREAMS IN THE GREATER YELLOWSTONE ECOSYSTEM

Beaver damming effects on the continuity of mountain streams can stem from isolated, episodic small beaver ponds, up to a nearly unbroken, long-lasting chain of ponds and floodplain wetlands with diffuse flow and few distinct channels. For the latter, the potential exists for substantial beaver-induced aggradation and sediment and water storage, but few data exist to test this concept. We are studying long-term beaver effects on small streams (basin area < 100 km²) in greater Yellowstone, over a variety of geomorphic settings. In northern Yellowstone (YNP) and Grand Teton (GTNP) National Parks, identification and ¹⁴C dating of beaver pond deposits show that damming is intermittent in space and time and strongly dependent on local geomorphic controls. In northern YNP, beaver dam sites with preserved pond deposits indicate a threshold of declining stream gradient with increasing basin area, showing a strong stream power control. About one-third of the total stream network length shows evidence for beaver-related aggradation in pond deposits or unusually thick and organic-rich fine-grained sediments. However, except in glacial scour depressions, net Holocene beaver-related aggradation is < 2.5 m, typically with 1-2 stacked pond deposit or wetland sequences. Beaver pond deposition is minimal during prolonged and severe droughts in the Holocene, with channel incision and flood gravel deposition on some streams. Along floodplains of larger streams, beaver often dam tributaries and side channels fed by valley-side springs and hyporheic flow. This avoids dam maintenance on powerful main channels and has little impact on their morphology, but enhances overbank sedimentation and floodplain wetlands. On larger Odell Creek, in Centennial Valley, Montana, active beaver dams are concentrated along middle reaches, where gradient and sediment caliber decline downstream on a mountain-front fan, but lower reaches are mostly confined between older terraces. Although most have minor effects on channel morphology and sediment storage, dams promote avulsion and may be especially effective in initiating channel shifts near the fan intersection point. Overall, beaver damming does not dominate the morphology of these greater Yellowstone streams, but contributes substantially to fluvial and riparian habitat diversity.