The harbour is a drowned valley (ria) formed at the end of the last ice age and is the estuary of several rivers, the largest being the Frome.

Dated at about 295 BC, the 10 metres (33 ft) Poole Logboat is one of the largest vessels of its type from British waters.

Following the harbour anti-clockwise, heading north-east passes the built up residential settlements of Poole including Lilliput and Parkstone (east).

Continuing anti-clockwise, heading west around the Harbour are the settlements of Upton and Wareham, as well as the outlet of the River Piddle.

Continuing anti-clockwise, now heading south are the majority of the islands within the Harbour as well as several small channels and inlets.

Continuing anti-clockwise comes back to the entrance to the Harbour and to Poole Bay, with Studland beach immediately south-east.

Lytchett Bay lies to the north of the Harbour and flows into it through a narrow channel near the edge of the suburb of Hamworthy.

Access to Holes Bay for vessels with an air draft greater than 2.1 metres (6.9 ft) is only possible when the bridges are lifted, which occurs several fixed times daily and sometimes on request.

[9] Holes Bay is the location of the Royal National Lifeboat Institution (RNLI) training school, attached to their Poole headquarters.

These islands include: The harbour lies on a band of weak gravel and clay which is easily eroded by the rivers and sea.

A relatively resistant chalk ridge ran continuously from the Purbeck Hills to the Isle of Wight, which the rivers could not break through.

About 7,000 years ago, increased erosion from the sea and the increased flow caused by the change in climate broke through the chalk hills, cutting the Isle of Wight off from the Isle of Purbeck and flooding what is now the Solent and Christchurch Bay, leaving Poole Harbour as the estuary of the Frome.

[13] Their duties include maintaining the shipping channels for the ferries and cargo vessels, enforcing harbour speed limits, improving port facilities and assisting with nature conservation.

[14] In November 2005 the main shipping channels into the harbour and the Port of Poole were dredged to accommodate modern ferries at all states of the tide.

Much of the north side of the harbour is a built-up area, including the town of Poole, and the conurbation which continues 10 miles (16 km) eastwards along the coast.

The mouth of the harbour is partially blocked by Sandbanks, a spit on the north, which is built up and part of Poole, and by Studland to the south, which is another important wildlife area.

The harbour is very shallow in places and has extensive mud flat and salt marsh habitats, as well as muddy and sandy shores and seagrass meadows.

Around Wareham Forest in the west this has been for commercial forestry, but on the southern shore the plantations conceal the Wytch Farm oil wells.

Three bird species occur in internationally important numbers: common shelduck, pied avocet and black-tailed godwit.

[18] On 23 April 2022, ospreys nesting in Poole Harbour produced an egg, the first in southern England in modern times.

However, the line is fenced off at the Poole Docks level crossing with a run-round loop provided at Hamworthy Park.