Your piece on the differing responses to wolf reintroduction in Yellowstone was a welcome change from the oversimplified accounts that have dominated media coverage (“Have returning wolves really saved Yellowstone?” HCN 12/8/14). One important factor was missing, even though it is likely to become the most critical one: climatic change.
Our University of New Mexico-based research on the long-term history of beaver damming and streams covers the specific drainages in the article, along with many other small streams in the dry lower elevations of Yellowstone and Grand Teton national parks. The data clearly show that during warm and dry intervals within the last few thousand years, beaver activity was markedly reduced –– in particular during the Medieval Climatic Anomaly of 900 to 1300 A.D., when prolonged droughts shrank lakes and streams and spurred severe forest fires in much of the West, including greater Yellowstone.
David Cooper pointed to Elk Creek’s exposed sediments as evidence that the now deeply gullied valley was a pond environment for thousands of years. Indeed, our analysis and dating of sediments at that site show that wet, silty beaver meadows persisted over much of the last 4,000 years. But that environment was disrupted by forest fires and ensuing gravel-depositing floods during the Medieval Climatic Anomaly, when Elk Creek and most other small streams in Yellowstone and Grand Teton lack evidence of beaver ponds, and several down-cut through former pond deposits. Likewise, floods following the 1988 fires greatly increased gullying. Also, incision is not ubiquitous at present; the Elk Creek site represents one of the most severe impacts in a spectrum that also includes channels with no down-cutting, despite beaver dam abandonment. Only about 30 percent of the length of small streams in Yellowstone’s northern elk winter range show clear evidence of past beaver damming, a reflection of habitat limitations, and substantial reaches (including parts of Elk Creek and Blacktail Deer Creek) have dated terraces showing down-cutting that began many hundreds of years before the current episode of dam abandonment and channel incision.
Climate has also played a role in changes since Yellowstone’s establishment in 1872. Following the elimination of wolves, a cooler and often wetter climate in the early 1900s combined with abundant willow and aspen and a lack of predators to assist beaver populations in rebounding from trapping. The Dust Bowl drought of the 1930s amplified the impact of browsing by burgeoning elk populations on beaver habitat.
But of most concern are the rising temperatures and severe droughts of the last few decades. Some streams where Warren found abundant beaver in the 1920s now run dry in summer, and the lack of flow cannot be attributed to loss of water storage by beaver damming, as they are fed directly by springs that are no longer flowing. Other studies have documented the recent drying of once-perennial ponds in northern Yellowstone, and the massive 1988 burns and subsequent large fires in the Northern Rockies have been linked to warming temperatures, much more so than fire suppression.
We are disinclined to think that some streams will “never” recover, as the article suggests; in the last millennium, incised channels have refilled and beaver meadows have returned as environmental conditions changed. But warming by greenhouse gases with long residence times in the atmosphere has the potential to desiccate greater Yellowstone’s small streams over many hundreds of years, with impacts that will resonate throughout water-limited ecosystems.
Grant Meyer and Lyman Persico
Albuquerque, New Mexico
This article appeared in the print edition of the magazine with the headline Yellowstone’s climate threat.
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