Why Does That Estuary Look Like a Drowned Valley? Because It Is One

During glacial periods a great deal of water is held on land as ice, so sea level falls by more than a hundred metres, and rivers extend their courses across the exposed shelf and cut valleys into it. When the ice melts, sea level rises and the sea floods those valleys from the seaward end inwards, filling them and reaching far inland along a course the river cut. The resulting inlet therefore has the shape of a river system rather than of anything the sea made, with a winding main channel, branching tributary arms and a bed that deepens gradually seawards. The last such flooding, following the end of the most recent glaciation, produced the ones that exist now.

Several features distinguish them from other coastal inlets:

• •A branching dendritic shape matching a river system
• •A winding rather than straight course
• •A V-shaped cross section with sloping rather than vertical sides
• •Depth increasing gradually towards the sea, with no shallow sill at the mouth
• •Tributary valleys entering as separate arms
• •Surrounding land dissected by the same river system above the water

The two are frequently confused and the difference is in what cut the valley. A fjord was carved by a glacier rather than by a river, which produces a U-shaped cross section with steep and frequently vertical sides, enormous depth, and a shallow sill of debris at the mouth marking where the glacier ended. The flooded river valley has none of those, being V-shaped, comparatively shallow and deepening seaward. The distinction is visible on any chart from the depth profile alone. Both are drowned valleys and both occur on formerly glaciated coasts, and the glacier version occurs only where ice reached the sea while the river version occurs wherever the sea rose over a river valley.

The mixing of fresh and salt water in a sheltered basin produces conditions that few species tolerate and those that do occur in enormous numbers. Salinity varies with the tide and with river flow, so an organism living there must handle a range rather than a fixed condition, which excludes most marine and most freshwater species and leaves a specialised community. Fine sediment settles in the calm water and builds mudflats rich in organic material, which support extraordinary densities of worms, molluscs and crustaceans. That food supply is why estuaries hold internationally important numbers of wading birds and wildfowl in winter. Several commercially important fish species use them as nurseries, and eels and salmon pass through on migration.

These inlets have been used as harbours for as long as there has been shipping, and the reasons are structural. They are sheltered from the open sea by their winding course and by the surrounding land, so water inside stays calm in weather that would close an exposed harbour. They are deep close to the shore, since the valley was cut by a river and not filled with sediment, so ships can come alongside without dredging. They reach far inland, which places a port close to the country it serves. And they have a river at the head supplying fresh water and inland transport. Plymouth, Falmouth, Sydney, Chesapeake Bay and a great many others sit in exactly this arrangement.

Falling sea level during a glaciation let rivers cut valleys across the exposed shelf, and rising sea level flooded them, producing inlets shaped like river systems with winding branching courses and V-shaped cross sections. A fjord differs by having been carved by ice, which leaves steep sides, enormous depth and a sill at the mouth. Shelter, deep water near shore and reach inland are why so many major ports occupy one.