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Regulation-induced channel gradation in the Peace River Ayles, Christopher P.

Abstract

River regulation imposes direct changes on flow and sediment delivery, producing a suite of downstream responses in channel morphology. On the Peace River in northern British Columbia and Alberta, the W.A.C. Bennett hydroelectric dam has reduced peak flows while leaving sediment load effectively unchanged. My research aims to identify systematic, regulation-induced patterns of channel gradation in the mainstem Peace and its tributaries below the dam. The significance of regulation within the natural variability of basin hydrology is assessed by comparing actual regulated river flows to simulated flows based on reservoir level fluctuations. Mainstem bed elevation changes are assessed from repeatedly surveyed cross-sections and specific gauge records, supplemented by analysis of channel planform change. Results show degradation to be minimal, due to the naturally armoured gravel bed and elimination of competent flows. The predominant pattern in the upper regulated reaches is one of aggradation below tributary confluences and other sediment sources. In the long term, the Peace River may be raising its proximal bed to compensate for a loss of sediment transport capacity since regulation. Backchannel abandonment and other planform changes appear to be occurring more slowly, and may be less important to river slope adjustment. Data from the lower river are few and inconclusive. Tributary gradation was investigated by means of air photo, field surveys and dendrochronology of young floodplains. These methods reveal a range of responses to regulation, including degradation, aggradation and no apparent change. Degradation due to reduced tributary base level appears to attenuate downstream as the Peace River flood is restored by unregulated tributary flows, though this trend is complicated by other factors such as tributary sediment supply, flood timing between tributary and mainstem, and ice activity. Aggradation due to tributary fan growth may mitigate degradation; it is a less prominent response, though it appears to predominate in the lower Smoky River. Regulation is a secondary effect in the tributaries, and its influence on gradation has been limited. On the mainstem, however, it is a primary change, and the resulting channel gradation will take a long time to complete.

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