The hydrological impact of the Parangana Dam on the Mersey River, Tasmania

By diverting the waters of the Mersey River into a neighbouring basin, the Parangana Darn changed the hydrological character of the downstream river. The flow records of three gauging stations, located at 10, 67 and 88 km below the dam, provide the basis for identifying changes. Intermediate discharges have decreased the most and even at the furthest downstream station are 50% lower than before. The flow extremes, at both ends of the scale, have been less affected, and particularly the less frequent discharges which can be augmented by overspill. Indeed the highest flood on record, with a recurrence interval of over 500 years, occurred in the post-dam period. One-day and 30-day minimum flows are markedly lower downstream of the dam, but below the Dasher, the main tributary of the Mersey, they seem to recover and have almost regained pre-dam levels by the mouth. The entry of unregulated tributaries in the middle section separates a much changed upstream hydrology from a downstream one able to mitigate the worst effects of the dam except at intermediate discharges. In the upstream part, the physical, chemical and biological character of the river could be adversely affected by the altered flow regime, particularly in those reaches immediately below the dam.