McSweeney, S.L.; Hart, D.E., Todd, D.J., and Kennedy, D.M. 2016. Changes in the Frequency and Duration of Closures of the Opihi Rivermouth Following Construction of Opuha Dam. In: Vila-Concejo, A.; Bruce, E.; Kennedy, D.M., and McCarroll, R.J. (eds.), Proceedings of the 14th International Coastal Symposium (Sydney, Australia). Journal of Coastal Research, Special Issue, No. 75, pp. 88–92. Coconut Creek (Florida), ISSN 0749-0208.Hapua are coastal lagoons found at the mouths of braided gravel rivers that experiee a semi-permanent connection with the ocean related to fluvial discharge and marine sediment deposition. Sustained closure is driven by low river flows and is associated with a decrease in water quality, the impedance of fish passage and flooding. On many Hapua, irrigation schemes have modified natural flow regimes, yet there is little field data on how this affects the dynamics of the lagoon. In this study we examine the Opihi rivermouth (New Zealand) which has a naturally seasonal low flow regime and a hydrology extensively modified through irrigation abstraction and dam construction Analysis of the entrance morphodynamics pre and post-dam construction indicates a decrease in the frequency and duration of rivermouth closures. This is due to the more constant flow regime maintained by the dam. Despite this, seasonal patterns of closure still persist over summer which illustrates the importance of marine processes in driving entrance condition. Three scenarios of natural closure are identified: summer low flows, fluvial slug deposition, and coastal storms. Closure duration was also observed to be extended when river flows are low, onshore wave energy is high and when the outlet channel is considerably offset.