Sunset over estuarine channel at low tide, Nelson Haven

Tidal estuaries are unique coastal environments, sheltered from wave action and home to many species of animals, plants and birds that are rarely found elsewhere on the coast. Estuaries are commonly associated with river mouths or enclosed bays and their sheltered nature means that they are efficient traps for fine sediment brought in by rivers or coastal currents. Estuaries are therefore being continuously filled in over time by the gradual buildup of sediment, meaning that, geologically speaking, they are relatively short-lived features of the coast, lasting usually only a few thousand years after the end of a sea level rise.

In New Zealand the last major sea level rise ended about 6500 years ago, following the end of the last Ice Age, and numerous estuaries were formed by the flooding of low-lying river valleys and embayments. There are many places in New Zealand where some of these estuaries have since been filled in by natural processes, forming low-lying wetlands or coastal plains that have commonly been drained and converted into farmland. Tasman Bay has many such environments, ranging from active estuaries such as Nelson Haven (in this photograph) through to the infilled (and now farmed) coastal plains around nearby Richmond and Motueka to the west.