THE MARINE FAUNA OF THE BLACKWATER ESTUARY 5
In the main body of the Blackwater (mid-channel, off Bradwell),
samples taken in 1963 showed that the salinity varied from
31.9 P.P.T. (April) to 34.8 P.P.T. (August) and the temperature
from -0.5° C (February) to 19.0° C (July). There are of course
many cases where both salinity and temperature range outside
of these figures, particularly in relation to wet and dry seasons
and extremes of summer heat and winter cold. For instance in
January 1963, the Blackwater was largely covered with ice so
that the water temperature had decreased to — 4.0 ° C in response
to minimum air temperatures as low as —11.0° C (West Mersea
22-24 January). At any one time there will also be variations in
salinity and temperature on the seashore and in the truly estuarine
parts of the Blackwater, compared with the figures at Bradwell.
Tidal variations are fairly large—the highest spring tide predic-
tion for Mill Creek, Tollesbury, in 1962 gave a range of 18.6 ft.,
and the smallest neap tide a range of 9.3 ft. The actual tidal
levels, however, may vary considerably from the prediction, with
the velocity and direction of the wind.
A considerable amount of data on the physical conditions and
chemical composition of the waters of the Blackwater has been
collected from 1960 to the present time and it is hoped that this
information will eventually be published.
The shores of the Blackwater and the Colne are low-lying, with
higher ground (below 50') reaching the water's edge at a few
places only, e.g., Cudmore Grove on Mersea Island. Otherwise
the shores are bounded by sea walls, saltings or shingle beaches.
The inlets which form the tidal waters of the two rivers originate
from the drowning of the lower river valleys, as a result of sub-
sidence of the coast, during isostatic readjustment after the last
glaciation. Originally the shore would all have been tidal marshes
with reed beds and saltings, but these have been reduced by
enclosure and drainage and by natural erosion. The erosion and
wave action result in large amounts of silt being carried in
the water at some times and many sheltered creeks are slowly
being silted up. Where erosion is outpacing deposition, the
physical features of the coast are changing markedly. Erosion
is particularly noticeable on the south shore of Mersea Island
and at Sales Point, Bradwell. The saltings have been eroded
away to leave furrowed platforms of alluvial clay which are being
slowly planed off and covered with a variable thickness of soft
mud, sand, stones and shells. Much of the coarser material is
carried up to high watermark forming steep banks. There appears
also to be a drift of this material into the mouths of the estuaries.
In a few places the earliest condition of the topography with
saltings coming into direct contact with the dry land, can still be
seen. Here there is a natural transition from halophytic to non-
halophytic vegetation, and the stumps of dead trees, still in situ
in the marsh, show the continuing encroachment of sea upon
land. Perhaps the best example of this is seen in the small
salting just west of Cudmore Grove on Mersea Island.