Geography
Definitions of the limits of the Severn Estuary vary. A narrower definition adopted by some maps is that the river becomes the Severn Estuary after the Second Severn Crossing near Severn Beach, South Gloucestershire and stretches to a line from Lavernock Point (south of Cardiff) to Sand Point near Weston-super-Mare. A wider definition is that the estuary extends upstream to Aust, the site of the Severn Bridge. The estuary is about 2 miles (3.2 km) wide at Aust, and about 9 miles (14 km) wide between Cardiff and Weston-super-Mare.
The estuary forms the boundary between Wales and England in this stretch. On the northern side of the estuary are the Caldicot and Wentloog Levels, on either side of the city of Newport; and, to the west, the city of Cardiff together with the resort of Penarth. On the southern, English, side, are Avonmouth, Portishead, Clevedon, and Weston-super-Mare. Denny Island is a small rocky island of 0.24 hectares (0.6 acre), with scrub vegetation, approximately three miles north of Portishead. Its rocky southern foreshore marks the boundary between England and Wales, but the island itself is reckoned administratively to Monmouthshire, Wales.
The estuary has one of the highest tidal ranges in the world — about 15 metres (49 ft). The estuary's funnel shape, its tidal range, and the underlying geology of rock, gravel and sand, produce strong tidal streams and high turbidity, giving the water a notably brown coloration.
West of the line between Lavernock Point and Sand Point is the Bristol Channel, which in turn discharges into the Celtic Sea and the wider Atlantic Ocean. The islands of Steep Holm and Flat Holm are located close to that line, in the middle of the estuary.
Sometimes the term Severn Estuary is used to include the tidal upstream stretch between Gloucester and Aust. During the highest tides on the upper reaches of this stretch, the rising water is funnelled up the estuary into the Severn bore, a self-reinforcing solitary wave that travels rapidly upstream against the river current.
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