The Severn Estuary is the mouth of the River Severn as it flows southwest into the open sea of the Bristol Channel. In addition to the Severn, three other major rivers empty into the estuary: the Wye, the Avon and the Usk.

The extent of the estuary stretches upstream from the tidal limit of the river at Gloucester Docks to a line running from Lavernock Point in South Wales to Sand Point in North Somerset. The outer limit of the estuary is marked by two rocky islands, Flat Holm and Steep Holm.

The estuary formed over the last 12,000 years as rising sea levels after the last ice age flooded a low-lying valley. Over time, the rivers that empty into the estuary deposited huge quantities of mud, sand and gravel creating a varied seabed of flats and bars, along with huge wetlands around the fringes. The reclamation of these wetlands, with embankments and ditches from the 1st Century CE onwards, created the Gwent (and Somerset) Levels.

The funnel shape of the estuary makes it unique in Britain and very rare worldwide. It creates the highest tidal range in Europe and one of the highest in the world at over 14 metres. As the ebb tide recedes, it reveals a huge intertidal area of saltmarsh, mudflats and sandbanks.

The shape of the estuary, and its southwest orientation make it very susceptible to storms sweeping in from the Atlantic Ocean. Storm surges, caused by a combination of high winds and low atmospheric pressure, can add 1-2 metres to tides. The famous Great Flood of 1607 that inundated the Levels, and which is commemorated at churches across the area, was probably caused by a high spring tide coinciding with a storm surge that overwhelmed sea defences.

The estuary has been vitally important for international trade and maritime navigation for many centuries. From the medieval period onwards, important ports developed at Bristol, Newport and Cardiff, along with many smaller landing places scattered along the coast, including Peterstone, Goldcliff and Redwick. The importance of the estuary, and the difficult conditions that prevail, is attested to by the many shipwrecks from all periods that litter the seafloor.

The Severn Estuary is internationally recognised for its important habitats. Much of the estuary is within a Special Area of Conservation (SAC). Large sections of the intertidal zone are designated as a Special Protection Area (SPA), Ramsar site and Site of Special Scientific Interest (SSSI).

Conditions in the estuary can be very changeable, with fluctuating salinity, strong currents and highly mobile sediments. The presence of high levels of organic matter in the mud provide food for animals that burrow, such as lugworms. However, organic matter also removes oxygen as it is digested by bacteria, restricting life to the surface levels. Fine silt particles can also be a problem for animals with delicate feeding or breathing mechanisms. This limits the range of bottom-dwelling (benthic) species. However, those that can survive in this harsh environment tend to occur in huge numbers.

These invertebrates support internationally important populations of waders, including Curlew (Numenius arquata), Redshank (Tringa totanus), Ringed plover (Charadrius hiaticula) and Grey plover (Pluvialis squatarola). The estuary also supports around 10% of the British wintering population of Dunlin (Calidris alpina) and is the single most important wintering ground for this species.

Seven species of migratory fish move through the estuary between the sea and rivers, including Atlantic salmon (Salmo salar), Sea trout (Salmo trutta) Common eel (Anguilla Anguilla), Sea lamprey (Petromyzon marinus), River lamprey (Lampetra fluviatilis), Twaite shad (Alosa fallax) and Allis shad (Alosa alosa).