Impacts of dam-regulated flows on channel morphology and riparian vegetation: a longitudinal analysis of Satsunai River, Japan |
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Authors: | Mayumi Takahashi Futoshi Nakamura |
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Institution: | (1) Graduate School of Agriculture, Hokkaido University, N9W9, Sapporo 060-8589, Japan |
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Abstract: | We examined the impacts of the Satsunai River Dam on the hydrology and development of riparian vegetation along the upper
and lower reaches of the Satsunai River downstream from the dam. We estimated frequency curves of the flood discharge during
the pre-dam (1976–1996) and post-dam (1997–2006) periods and simulated the flood frequency at sampling points within sites
under pre-dam, post-dam and dam-removal (using the pre-dam flood discharge and post-dam cross-sections) scenarios. Changes
in channel morphology and land cover were investigated by analyzing aerial photographs. Our results indicate that the 20-year
flood at the upper site decreased substantially (from 599 to 271 m3/s) after dam operation, while that of the lower site decreased slightly (from 1025 to 977 m3/s). Within the upper site, the proportion of >20-year return periods increased considerably (from 31.0 to 48.6%) while the
proportion of 1- to 20-year return periods decreased (from 30.5 to 8.9%) after dam operation. Flood frequency results for
the dam-removal scenario were similar to those for the pre-dam period, suggesting that a return to pre-dam discharge rates
would restore the pre-dam distribution of flood frequency at the upper site. Within the lower site, however, the distribution
of flood frequency varied little between the pre- and post-dam scenarios, because tributary inflows between the sites mitigated
the impacts of dam-regulated flows. Land cover types were associated with flood frequency at both sites. The reduced flood
frequency of the upper site resulted in increased area of riparian vegetation and decreased area of active channel. |
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