Environmental

Coasts, estuaries, sea levels

It seems natural to think of an estuary in terms of the river flowing into it, to see it as the end of the

river, with a boundary somewhere offshore where the river meets and merges with the sea. Stand on

a headland overlooking an estuary and this is how it looks, but the picture is misleading. An estuary

is more accurately described as an arm of the sea that extends inland and into which a river flows. An

estuary is dominated by the sea rather than its river, and many estuaries are in fact ‘rias’, or ‘drowned

river valleys’, old river valleys which were flooded at some time in the past when the sea level rose.

The estuaries of south-west England are good examples of rias. In several cases, such as the Camel

in north Cornwall, before the marine transgression that began about 10300 years ago the sea was 36

m below its present level (the sea is still rising at about 25 cm per century), and gently undulating

land, with hills formed from igneous intrusions through Devonian slate which survive now as offshore

islands, extended up to 5 km from the present coast. This land was blanketed with mixed deciduous

forest. Remnants of the forest have been found on the sea bed at several points along the coast and its

botanical and faunal composition determined (JOHNSON AND DAVID, 1982).

Sea levels change and at various times in the past they have been both higher and lower than they are

today, and they are changing still. During glacial periods (ice ages), sea levels fall, because the

volume of the oceans decreases as water evaporated from them accumulates in ice sheets. As the

weight of ice depresses the land beneath it sea levels rise; as the ice sheets melt they also rise; and as

land depressed by the weight of ice rises again when the ice has melted they fall. There is clear

evidence in many places that sea levels were much lower at some time in the past. Raised beaches

can be found that are several metres above the present high-tide level. These are areas of approximately

level ground, nowadays usually vegetated, containing large numbers of shells of marine organisms.

They can have been produced only by the movement of waves and tides over them, at a time when

they formed the shore; they are ancient beaches now some distance from the sea.

The sea bed at the mouth of the Camel estuary is mainly sandy, with sand bars, and there are many

sandy beaches along the adjacent coast. Sand consists primarily of quartz grains weathered and eroded

from igneous rocks inland and transported by the river. They are deposited at the mouth of the estuary,

then transported further by tides and sea currents. As they move they become mixed with varying

amounts of sea shells, most of which are crushed to tiny fragments through being battered by harder

stones, producing a beach material with a

relatively high calcium carbonate content;

this was formerly used by farmers as ‘lime’,

to raise the pH of their soils.

Sand that has been transported many miles

by river is deposited where fresh water and

sea water meet. Because sea water is

denser than fresh water the two do not mix

readily and tend to flow in separate

channels. The configuration of these

channels is determined by the topography

of the estuary itself; they may flow side

by side or form a wedge, in which fresh

water rises over the sea water. On an

incoming tide, freshwater and seawater

currents often flow in opposite directions

and marine fish can move considerable

distances inland by keeping to the saltwater channel.