Christchurch Harbour is a natural harbour in Dorset, on the south coast of England. Two Rivers the Avon and the Stour flow into the Harbour at its northwest corner. The harbour is generally shallow and due to the tidal harmonics in the English Channel has a double high water on each tide. On the north side of the harbour, east of the River Avon are Priory Marsh, and to the east of this Stanpit Marsh, a Local Nature Reserve. To the west side of the harbour are Wick Fields, the southern flank of the harbour being bounded by Hengistbury Head, a prominent coastal headland.
Christchurch Harbour contains large areas of salt marsh and is protected by a sandbar known as Mudeford Spit which has fine sandy beach on both sides of a walkway lined with beach huts. The harbour is protected by a natural headland (Hengistbury Head) at the start of the sandbanks, and is a special site for sand martins which nest annually in the sandy cliffs. The harbour is only accessible to shallow draught boats drawing up to 4 feet (1.2 m) due to the sandbars at the entrance. The entrance, known as the Run, has Mudeford Quay on one side and the spit on the other. Considerable tides flow here: up to 6 knots (11 km/h; 6.9 mph) during spring tides. The harbour is a protected wildlife refuge and is home to large populations of swans, waders and other bird life. On the south side, the harbour is enclosed by Hengistbury Head which was the site of the earliest settlement here dating back to the Bronze Age. The landward end of the headland has a bank and ditch known as Double Dykes, built in about 700 BC, to protect the ancient settlement.
The harbour was formed around 7000 years ago when the sea level rose at the end of the last Ice Age. Previously the area which was many miles from the open sea was inhabited by Stone Age hunters. Archaeological finds dating from 12,500 year BP have been made on Hengistbury Head and Flints dating as much as 250,000 years BP have been found in the Bournemouth area. The Bluestones used at Stonehenge may have been transported via the harbour and the River Avon (2550 BCE). It is suggested that there may have been an ancient causeway usable at low water running from Double Dykes on the south shore to Tuttons Well located on the north shore near Stanpit village