SURVEY AND COMPARISON 157
and begins to fill about 11/2 hours before high tide. By just
over an hour after high tide, it is empty, except for a small
channel on the south side. The movement of water in the
creek is, therefore, rapid. Water enters and leaves the
creek at both east and west ends. The marsh extends from
the northern margin of this creek to the base of the sea wall,
and is very flat. In a few areas there are differences in level,
and corresponding differences in the flora are found here.
The main marsh is dissected by creeks which branch off
Creek 14, and the general level of the marsh is 4-5 feet above
the beds of the largest of these channels. A very short time
before high tide, these smaller creeks begin to fill, and they
are actually full for about '20 minutes only. Some of the
innermost creeks are not filled at every neap tide, but there
is water in them at each tide.
Since the main part of the marsh is at one fairly high level,
there are two main factors which affect the vegetation.
These are (i) salinity;
(ii) rapidity of drainage.
(i) Salinity. This has been calculated by means of
titrations of sea and creek water, and of solutions
prepared from soil specimens. Standard silver nitrate
solution was used throughout, and details of the
method and calculation are given separately. The
following table shows the results for specimens from
the main salt marsh.
Specimen. Salinity.
Sea Water ...... 17.500 grains chloride/litre (i.e., 1.75%)
Creek Water ...... 17.100 grams chloride/litre (i.e., 1.71%)
Soil round roots, Aster .035 grams chloride/gram
Tripolium L. ... soil ... ... (i.e., 3.50%)
Soil round roofs, Suaeda .034 grains chloride/gram
maritima (L.) Dum. soil ... ... (i.e., 3.40%)
(ii) Rapidity of Drainage. When the small creeks are
filled at high tide, the whole of the marsh surface is
wetted, and at the highest tides the surface is
covered to a depth of several inches. As soon as the
level in the creeks falls, water on the marsh begins
to run back into the channels. Excess water and that
from the margins close to the drainage channels,
moves away rapidly because of the pull of the ebbing