Evidence from numerical modeling of the M-2 tide, supplemented by geological data, reveals marked changes in the tidal and sedimentary regimes of Cook Strait over the last postglacial rise in sea level. Approximately 18,000 years B.P., when sea level was approximately -113 m, the strait was reduced to a narrow bay by the emergence of a land bridge connecting the North and South islands of New Zealand. High sediment supply and low, tidally induced bottom stress encouraged widespread deposition of fine sediment except at the bay mouth (the Narrows) where localized high stress resulted in the transport of sediment to the deep ocean via Cook Strait canyons. At 17,000 years B.P. (approximately -88 m), the bay reached maximum size and was near resonance with the oceanic tide resulting in high tidal amplitudes, bottom stress, and sediment movement at the Narrows and bay head. The land bridge was breached and Cook Strait was formed about 15,000 years B.P. (approximately -75 m). Both Narrows and breach were subject to high bottom stress, whereas the intervening area was a zone of convergent low stress and fine sediment deposition. By 11,000 years B.P. (approximately -40 m) Cook Strait was well established and had a tidal response similar to the modern strait. The rising sea levels inundated remnants of the land bridge to form the broad continental shelf approaches to Cook Strait. Today, overall tidal stress is reduced, and mud deposition prevails except on the inner shelf where periodic forcing by wind-induced currents maintains a sandy substrate, and in the Narrows where a tidal phase difference of approximately 120 degrees maintains a zone of high sediment movement.
