Estuarine channel at low tide, Nelson Haven

Nelson Haven is a large tidal estuary extending north of Nelson city, enclosed by the 13 kilometre-long natural breakwater known as the Boulder Bank. Formed from rocks eroded from the cliffs of Mackay Bluff, the Boulder Bank has gradually extended towards present-day Nelson over the last 6,000 years as northerly storm waves have washed rocks southward. The Boulder Bank is regarded as an unusual coastal landform, and is also the largest natural breakwater in the world.

Nelson Haven experiences a large tidal range compared to most of the rest of New Zealand, with spring tides exceeding 4.5 metres. Behind the Boulder Bank extensive areas of mudflats lie exposed between tides – during extreme low tides Nelson Haven is in fact almost completely empty. Tidal currents have led to a well-developed network of channels on the mudflats, individual channels taking on classic smooth, sinuous forms such as the one shown in this photograph.