During the highest tides, rising water is funnelled up the estuary into the Severn bore, a self-reinforcing solitary wave that travels rapidly upstream against the river current.

The Ordnance Survey refers on its published mapping to a section of the estuary seaward of the two estuarine motorway crossings as the Mouth of the Severn.

[13] The tidal range results in the estuary having one of the most extensive intertidal wildlife habitats in the UK, comprising mudflats, sandflats, rocky platforms and islands.

These form a basis for plant and animal communities typical of extreme physical conditions of liquid mud and tide-swept sand and rock.

[15][16] The estuary is recognised as a Special Protection Area (SPA) under the EC Directive on the conservation of Wild Birds.

[21] The Severn Estuary SSSI original designation involves the then counties of Somerset, Avon and Gloucestershire in England, and Gwent and South Glamorgan in Wales.

The Severn Estuary SSSI designation overlaps individual site designations for separate sites in Avon (Spring Cove Cliffs, Middle Hope, Portishead Pier to Black Nore, Aust Cliff), Gloucestershire (Purton Passage) and South Glamorgan (Penarth Coast).

[22] Both SSSI citations provide detail of the geological and biological interest and of particular note is the international importance for wintering and wading birds of passage,[1] and of estuarine habits of outstanding ornithological significance.

In April 2006 the Welsh Assembly approved the idea of utilising the tidal power, but the RSPB has raised serious concerns about the effect on the mud flats, that have European Environmental protection status, and the UK government Energy Review published later in the year did not endorse the scheme.

John Hutton, Secretary of State for Business, Enterprise and Regulatory Reform, announced a further feasibility study on 25 September 2007.

[25] The proposal for a hydro-electric barrier to generate 8.6 GW and meet five percent of Britain's power needs, is being opposed by some environmental groups.

[26][27] The Severn Estuary Partnership (SEP) was set up in 1995 as an independent initiative to focus the activities of local government, statutory authorities and interested parties such as farmers and fisherman.

Its stated aim is To bring together all those involved in the development, management and use of the Estuary within a framework which encourages the integration of their interests and responsibilities to achieve common objectives.

The archaeological resource within the estuary is under threat from natural processes such as coastal erosion, exacerbated by the high tidal range and strong tidal currents, and from threats such as ongoing development pressure along the shoreline, marine aggregates extraction and new coastal defensive and realignment measures as well as proposed major infrastructure projects.