HUMAN IMPACTS TO FLUVIAL SYSTEMS IN YELLOWSTONE NATIONAL PARK: HISTORICAL PERSPECTIVE AND TRAJECTORY FOR THE FUTURE
A more complex impact to small streams began with beaver trapping in the early 1800s. After trapping bans and wolf extirpation (the beaver’s main predator) by park managers, beaver rebounded by the 1920s, aided by an unusually wet climate. However, elk competition with beaver for willow and aspen also increased markedly in the absence of wolves. Combined with drought in the 1930s, scarce food and dam resources resulted in beaver dam abandonment on virtually all small streams in northern Yellowstone by the 1950s. Some infer that abandonment caused widespread channel incision and drying of floodplains, but only ~9% of reaches incised after the mid-1900s loss of beaver dams, locally exacerbated by floods after the 1988 fires. Many remain unincised or began downcutting earlier in the Holocene. Wolf reintroduction has aided willow recovery, yet beaver recolonization has been hampered by still-sparse willow and aspen on some reaches and recent extreme droughts causing ephemeral flow in some small streams that hosted beaver in the 1920s. Droughts that restrict streamflows and promote postfire debris flows, floods, and sediment loading have visited Yellowstone throughout the Holocene, but will increase in severity and duration with anthropogenic warming, limiting prospects for beaver recolonization and increased riparian productivity. Compared to regional streams dammed, canalized, and dewatered for irrigation, human impacts to Yellowstone streams remain minor, but they will not be spared the effects of rising temperatures.