The Irrigation Effect: How River Regulation Can Promote Some Riparian Vegetation
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River regulation impacts riparian ecosystems by altering the hydrogeomorphic conditions that support streamside vegetation. Obligate riparian plants are often negatively impacted since they are ecological specialists with particular instream flow requirements. Conversely, facultative riparian plants are generalists and may be less vulnerable to river regulation, and could benefit from augmented flows that reduce drought stress during hot and dry periods. To consider this ‘irrigation effect’ we studied the facultative shrub, netleaf hackberry (Celtis reticulata), the predominant riparian plant along the Hells Canyon corridor of the Snake River, Idaho, USA, where dams produce hydropeaking, diurnal flow variation. Inventories of 235 cross-sectional transects revealed that hackberry was uncommon upstream from the reservoirs, sparse along the reservoir with seasonal draw-down and common along two reservoirs with stabilized water levels. Along the Snake River downstream, hackberry occurred in fairly continuous, dense bands along the high water line. In contrast, hackberry was sparsely scattered along the free-flowing Salmon River, where sandbar willow (Salix exigua), an obligate riparian shrub, was abundant. Below the confluence of the Snake and Salmon rivers, the abundance and distribution of hackberry were intermediate between the two upstream reaches. Thus, river regulation apparently benefited hackberry along the Snake River through Hells Canyon, probably due to diurnal pulsing that wets the riparian margin. We predict similar benefits for some other facultative riparian plants along other regulated rivers with hydropeaking during warm and dry intervals. To analyze the ecological impacts of hydropeaking we recommend assessing daily maxima, as well as daily mean river flows.
KeywordsCeltis reticulata Hells canyon Hydropeaking Salmon river Snake river
This research was supported with funding from Idaho Power Company (IPC), the Natural Sciences and Engineering Research Council of Canada, Alberta Innovates, and Alberta Environment and Parks. We extend thanks to Robert Simons of Simons and Associates of Colorado, and Gary Holmstead, Toni Holthuijzen, Frank Edelman, and Allen Ansell of IPC for their insightful discussions, Karen Zanewich for assistance with data analyses and graphics and two anonymous reviewers for their very helpful advisements.
Compliance with Ethical Standards
Conflict of Interest
The authors declare that athey have no conflict of interest.
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