Overview · eight British estuaries

Where the sea meets the rivers

Twice a day the sea flows up Britain's rivers, sometimes for more than 100 km. Where it meets fresh water lies one of the most productive, most used and most threatened landscapes in the country: the estuary.

163estuaries in the UK¹

~⅓of Western Europe's estuarine habitat¹

~16.5 cmUK sea-level rise since 1900⁹

5.2 Mtorganic carbon stored in Britain's saltmarshes⁶

How a tidal river works

An estuary is a partly enclosed stretch of coast where seawater mixes with fresh water from rivers.¹ The tidal limit is the furthest point upstream that the tide reaches; it is often fixed by a weir or lock, such as Teddington on the Thames or Howley Weir on the Mersey. The tide travels much further than the salt. Seawater is denser than river water, so salt pushes upstream along the bottom while fresh water flows out over the top, and the mixing zone shifts with the seasons. High river flow pushes the salt seaward; in a dry summer it creeps inland.²

Scientists divide the gradient by salinity, in parts per thousand: fresh water below 0.5, then oligohaline (0.5–5), mesohaline (5–18), polyhaline (18–30) and full seawater above about 30.³ On this map, "brackish" covers the middle of that range. Where fresh and salt water meet, fine sediment is trapped and concentrated into a turbidity maximum, which is why so many British estuaries are brown with mud.⁴

Shape matters. In a long, funnel-shaped estuary the incoming tide is squeezed into ever narrower and shallower water, so its range grows inland. That is why the Severn, with a range of up to about 14 m, has the second-highest tides in the world, and why it, the Mersey and the Trent can produce tidal bores, waves that run upstream on the flood tide.⁵

Why estuaries matter for nature

Almost half of Britain's coastline lies within estuaries, and the UK holds about a third of the estuarine habitat of Western Europe.¹ Their mudflats are among the most productive habitats on Earth, packed with worms, snails and shellfish. Each winter they feed internationally important numbers of waders and wildfowl from the Arctic; the Thames, Humber, Mersey, Severn and Forth each average tens of thousands of waterbirds.⁷ Estuaries are nursery grounds for fish such as sea bass, and the only route between sea and river for salmon, sea trout, eel, lamprey, shad and smelt.⁸ Saltmarshes also store carbon: Britain's marshes hold about 5.2 million tonnes of organic carbon, mostly in their soils.⁶

Many of these species are in trouble. The European eel is critically endangered, with glass-eel recruitment a tiny fraction of what it was in the 1970s, and England's salmon stocks are at their lowest on record.⁸ Sparling, or smelt, now survives in Scotland only in the Cree, the Forth and the Tay.⁸

Why estuaries matter for people

Britain's great cities and ports grew on tidal rivers: London, Liverpool, Hull, Newcastle, Glasgow, Edinburgh, Bristol and Dundee. In the industrial age the same rivers became drains. By the mid-20th century the tidal Thames, Mersey, Tyne and Clyde were so short of oxygen that fish could not pass. Their recovery, driven by sewage treatment and the decline of heavy industry, is one of the great environmental success stories of the past fifty years, although storm overflows still spill untreated sewage hundreds of thousands of times a year in England.¹⁰

A rising sea

Sea level around the UK has risen about 16.5 cm since 1900, and the rate has roughly doubled in recent decades.⁹ By 2100 it could rise by around half a metre to more than a metre at London, depending on global emissions.¹¹ Where seawalls are fixed, mudflats and saltmarsh are squeezed out between rising water and hard defences. Increasingly, the answer is to give space back: at sites such as Alkborough on the Humber and Steart on the Severn, embankments have been breached to let the tide create new marsh that absorbs floods and feeds birds.¹²

Eight rivers, eight characters

Thames
A capital's tidal river, from biologically dead to over 115 fish species. Severn
A 14 m tidal range, the Severn bore and the return of the shad. Mersey
From Britain's most polluted estuary to salmon at Warrington. Humber
A fifth of England drains through one muddy funnel. Tyne
From coal and shipyards to England's top salmon river. Clyde
Shipbuilding river, a sharp line between fresh and salt at Glasgow. Forth
A long firth of mudflats, bridges and Scotland's last sparling. Tay
The UK's mightiest river by flow, fresh almost to the sea.

Sources

UK tidal rivers legend

UK tidal rivers