River channel dynamics following extirpation of wolves in northwestern Yellowstone National Park, USA

Historical records indicate that gray wolves (Canis lupus) were extirpated from the upper Gallatin River Basin in the early 1900s. Following the removal of these large carnivores, elk (Cervis elaphus) began to increasingly browse streamside vegetation in the winter range, causing widespread loss of formerly extensive willow (Salix spp.) communities. Historical aerial photographs and chronosequences of ground photographs were used to characterize general changes in vegetation and channel morphology over time. In August of 2004, riparian vegetation and channel cross-sections were surveyed along three reaches of the upper Gallatin River. Reach A was located upstream of the elk winter range (control reach) whereas reaches B and C (treatment reaches) were located within the elk winter range. Willow cover on floodplains averaged 85 per cent for reach A, but only 26 per cent and 5 per cent for reaches B and C, respectively. The average return period of calculated bankfull discharges was 3·1 yrs for reach A but increased to 32·4 yrs and 10·6 yrs for reaches B and C, respectively. The long-term loss of streamside vegetation allowed channels to generally increase in hydraulic capacity (via increases in width, incision or both) and decrease their hydrologic connectivity with adjacent floodplains. This study documents, perhaps for the first time, the resultant impacts to riparian vegetation functions and stream channel characteristics following the extirpation of a large mammalian carnivore. Copyright © 2006 John Wiley & Sons, Ltd.

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