Basics of Environmental Science

Earth Sciences / 35

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 salt-

water channel.

As the fresh water is forced to rise, it loses energy, and the quantity of material a river can transport,
known as its ‘traction load’ or ‘bed load’, is directly proportional to the energy with which it flows.
This depends in turn on such factors as the potential gravitational energy causing the water to flow
(essentially the height of the river source above sea level), the gradient of the channel, and the
amount of friction caused by contact with the banks and bed (SMALL, 1970, pp. 34–41). As the
river water rises and loses energy, the sand grains sink, falling through the underlying salt water and
on to the bed. Figure 2.8 shows what happens and how over time the deposition of sand can lead to
the formation of a bar at the estuary mouth.

Sand does not remain static on the sea bed and material for the growth of a bar is also provided by
sand being carried landward or along the shore by tides and sea currents. The sea water also loses
energy as it pushes against the fresh water, and the sand it carries is again deposited.

Sand grains are much larger and heavier than the particles of silt rivers also carry. Silt particles are
2–60 micrometres (μm) in diameter, sand grains 60–2000 μm (in the British standard classification;
in the widely used Udden-Wentworth classification they are 4–62.5 μm and 62.5–2000 μm
respectively). Ordinarily, large particles would be expected to settle first and small ones later, but in
an estuary the opposite occurs. Mudbanks, composed of silt, still smaller clay particles and, mixed
with them, organic molecules from the decomposition of the waste products and dead bodies of
biological organisms, form inland of the sand banks. Flocculation is the process responsible for this
phenomenon. Many of the very small particles carry an electrical charge owing to the presence of
bicarbonate (HCO
3

• ), calcium (Ca2+), sulphate (SO
  4

2-), and chlorine (Cl-) ions. In the boundary zone

where fresh and salt water meet, these particles encounter chlorine, sodium (Na+), sulphate, and
magnesium (Mg2+) ions, which bond to them and attract more silt particles, so the material forms
clumps larger and heavier than sand grains, and these settle. The organic material mixed with them
provides rich sustenance for bacteria and, closer to the surface, burrowing invertebrate animals,
which provide food for wading birds. The environment is harsh because the salinity of the water
varies widely, so although only a restricted number of species can regulate their osmosis well enough
to survive in the mud, those which succeed do so in vast numbers. Estuarine waters may also be
enriched by a ‘nutrient trap’, where the current pattern causes dissolved plant nutrients to be retained

Figure 2.8 Deposition of sand and formation of an
estuarine sand bar