The Marlborough Sounds is a system of drowned river valleys, which were formed after the last ice age around 10,000 years ago.

Pelorus Sound has a main channel which winds south from Cook Strait for about 55 kilometres (34 mi), between steeply sloped wooded hills, until it reaches its head close to the Havelock township.

Pelorus has several major arms, notably Tennyson Inlet, Tawhitinui Reach, Kenepuru Sound and the Crail/Clova/Beatrix Bay complex.

Most of the settled places are hard to reach overland, and are serviced by the Pelorus Express, a mail boat which does three different weekly runs from Havelock.

According to Ngāti Kuia oral tradition, their founding father, a descendant of Kupe, came to the South Island in his waka Te Hoiere.

In 1864, gold was discovered in the Wakamarina Valley and, for a brief time, Havelock became a boom town as several thousand prospectors flooded the area.

[7] During World War II, coastal fortifications were constructed on Maud Island to protect Cook Strait, and the entrance to the sound.

[11] A New Zealand sea sponge, Mycale hentscheli, which grows in Pelorus Sound, may hold the key to an anti-cancer drug.

Marine farmers are helping Victoria University of Wellington and NIWA develop a method for growing the sponge on an existing mussel farm.

been replaced with a modified Japanese longline system, where mussel spat is packed into stockings and suspended from parallel rows of looped ropes, supported by buoys.

[15] Heavy rain can result in high sediment-levels in rivers and land runoff, and can contain bacterial contamination from livestock.

[15] Applications to increase mussel farms in Admiralty Bay have been successfully opposed on the grounds that the area is a special feeding site for the dusky dolphin.

A claim has been made, and a study[16][17] carried out in New Zealand, that marine farms may affect the feeding ability of Dusky dolphins which round up shoals of schooling fish such as pilchard.

Rain falling directly on the surface of the sound provides another 14 cubic metres per second (490 cu ft/s), and the remainder drains from adjacent land via small streams.

[19] This freshwater input corresponds to a depth of 10.8 metres (35 ft) of water distributed over the area of the sound per year.

Other tidal constituents, notably N2 (semi-diurnal, generated by the ellipticity of the Moon's orbit), introduce further modulation, meaning that the spring-neap cycle is not exactly repeated.

(These values are calculated from a sea level time series, July 1997 – February 1998, at Pelorus Sound entrance.)

The wind stress also generates turbulence, which causes the surface fresh water to be mixed down and results in a brackish layer typically 5 to 15 metres (16 to 49 ft) thick.

This is not simple, uniform tidal flow in and out of the bay because its amplitude does not follow a spring-neap cycle and there is a phase difference of several hours between the surface and the bottom.