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is below 50 per cent. during most of the summer, a minimum being reached in the neighbourhood
of the outfalls. Below Greenhithe the quality of the water rapidly improves and beyond the
Nore practically fully aerated water is usually found.
II. Factors affecting the river—
(i.) The progressive deterioration of the upper tideway appears to be due to local causes
rather than to the Council's operations. It does not seem to be due to the immediate effect
of up-river pollution, but rather to the inoffensive oxidation of organic matter in the mudbanks
exposed to air at low-water but stirred up by the rising tide.
(ii.) The summer condition of the important commercial area area London Bridge to
the outfalls is less satisfactory. Results of examination of daily samples of river water
collected at the outfalls, at both high and low water, point to a falling off of dissolved oxygen
since the year 1911, which was remarkable both for being hot and dry and that also in which
the total effluent discharged from the two outfalls reached a maximum.
(iii.) The influence of certain natural and other factors upon the river generally:—
(а) Fresh water flow was much higher during the summers 1911-1918 than in the
preceding summers of this century. The average daily flow was below 500 million gallons
in 32 months from 1901-10, in 4 months in 1911, and in only 7 months from 1912-1918.
Other data referring to flow confirmed the existence of relative dryness in the earlier
years.
(b) The temperature differences in the water during the summers of the two periods
were not of such an order as to suggest that greater biochemical activity caused the more
marked deterioration in the later summers.
(c) An important change began to take effect in the neighbourhood of 1911. The
Port of London Authority carried out an extensive and necessary dredging programme
which must have had, inter alia, two effects—(1) much matter must have been brought
up from the river bed which required oxidation; (2) the increased tidal flow, due to
the increased volume of the river, must have led to the agitation and commingling with
the water of mud which would otherwise have been undisturbed. If the view be accepted
that low aerations are, in part at least, attributable to dredging, then, in so far as this has
led to the oxidation of matters which unoxidised would be a potential source of nuisance,
its effect must have been on the whole good although obtained at the expense of an
apparent deterioration in the summer condition of the river.
(d) It might be suggested, in regard to the treatment of sewage, that the omission
of " chemical " treatment was a possible cause of low aerations, but as these were observed
when chemical treatment was carried out as well as when it was omitted this view cannot
be maintained.
(e) The amount of effluent discharged into the river from the outfalls has increased
almost continuously up to 1911, when a maximum seems to have been reached.
The relative importance of the various factors suggested above has not yet been determined,
partly owing to the very complicated nature of the problems involved, but largely owing to the fact
that some of these factors, e.g., those dependent on meteorological conditions, cannot be controlled
but only observed, and an adequate study of a series of observations extending over a number of years
is necessary.
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