Abstract
In mountain braided rivers, extreme flow variability, floods and high flow pulses are fundamental elements of natural flow regimes and drivers of floodplain processes, understanding of which is essential for management and restoration. This study evaluated flow dynamics and invasive vegetation characteristics and changes in the Ahuriri River, a free-flowing braided, gravel-bed river in the Southern Alps of New Zealand’s South Island. Sixty-seven flow metrics based on indicators of hydrologic alteration and environmental flow components (extreme low flows, low flows, high flow pulses, small floods and large floods) were analyzed using a 48-year flow record. Changes in the areal cover of floodplain and invasive vegetation classes and patch characteristics over 20 years (1991–2011) were quantified using five sets of aerial photographs, and the correlation between flow metrics and cover changes were evaluated. The river exhibits considerable hydrologic variability characteristic of mountain braided rivers, with large variation in floods and other flow regime metrics. The flow regime, including flood and high flow pulses, has variable effects on floodplain invasive vegetation, and creates dynamic patch mosaics that demonstrate the concepts of a shifting mosaic steady state and biogeomorphic succession. As much as 25 % of the vegetation cover was removed by the largest flood on record (570 m3/s, ~50-year return period), with preferential removal of lupin and less removal of willow. However, most of the vegetation regenerated and spread relatively quickly after floods. Some flow metrics analyzed were highly correlated with vegetation cover, and key metrics included the peak magnitude of the largest flood, flood frequency, and time since the last flood in the interval between photos. These metrics provided a simple multiple regression model of invasive vegetation cover in the aerial photos evaluated. Our analysis of relationships among flow regimes and invasive vegetation cover has implications for braided rivers impacted by hydroelectric power production, where increases in invasive vegetation cover are typically greater than in unimpacted rivers.
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Acknowledgments
The authors gratefully thank Chris Woolmore, the Department of Conservation Project Manager for PRR, for provision of most aerial photographs and assistance with the study. The authors also thank Luke Javernick, PhD candidate in the Department of Civil and Natural Resources Engineering at the University of Canterbury, for provision of the 2011 aerial photos and assistance with field work and reconnaissance. Financial and in-kind support for the project was provided by the University of Canterbury and Department of Conservation.
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Caruso, B.S., Edmondson, L. & Pithie, C. Braided River Flow and Invasive Vegetation Dynamics in the Southern Alps, New Zealand. Environmental Management 52, 1–18 (2013). https://doi.org/10.1007/s00267-013-0070-4
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DOI: https://doi.org/10.1007/s00267-013-0070-4