The harbour is generally shallow and due to the tidal harmonics in the English Channel has a double high water on each tide.

The varied habitats include saltmarsh, wet meadows, grassland, heath, sand dune, woodland and scrub.

[1][2] The harbour was formed around 7000 years ago when the sea level rose at the end of the last ice age.

Despite this there was considerable two way trade with both British and Foreign Ports, it then declined as a result of the Roman invasion of France in 56 BCE.

In 1664 The River Avon Navigation act was passed to again enable vessels to travel as far as Salisbury and reestablish it as a port as in medieval times.

In the 1930s there were three "tea boats" providing refreshments to holidaymakers and at least another five houseboats in the harbour these were all abandoned and then wrecked during World War II.

The sedimentation of the Harbour has not therefore been substantially affected in the way most other south coast estuaries have by the spread and die back of this species.

Extensive Phragmites reed beds occupy tidal creek margins and areas of higher marsh, where they front wet grazing meadows.

The Royalty Fishery Title was conferred by Royal Grant during Queen Mary's reign and was renewed by Elizabeth I.

The fishery was then sold to a Mr Thyrwit Walker in 1865, and it was purchased by West Hampshire Water Company in 1929.

From Tuckton Bridge, the two main sections of the harbour shore are Wick and Hengistbury Head, which are in Southbourne a suburb of Bournemouth.

The harbour proper begins around Wick Fields, a reed marsh and part of the area of importance for nature conservation.

Hengistbury Head on the south shore was threatened during the nineteenth century by the mining of ironstone doggers which dramatically increased erosion.

The silt washed into that part of the harbour threatened the ecology and to prevent this, the Quarry Pool was created on the headland by building a dam in 1976.

Holloway's Dock cuts into the land before the shore reaches Mudeford Sandbank which juts northwards towards The Run.

It was formed as the result of action and deposition of material from the rivers Stour and Avon as they meet with the salt water within Christchurch Harbour.

[23] It is a Site of Special Scientific Interest (SSSI) and an important nature reserve of about 65 hectares (160 acres), combining both freshwater and saltwater habitats.

A circular path on the marsh uses a prototype Bailey bridge to cross Mother Siller's Channel.