ON THE OCCURRENCE OF THE GUPPY 187
maintained even during periods of high river flow since any flood
waters from the River Lee are diverted over Lee Bridge Weir
some 450 metres upstream of the Power Station's outfall at Grid
Ref.: TQ 356866. For most of the year, however, virtually the
entire flow in the River Lee is abstracted for cooling water. This
is considered to be important as it means that river water is
constantly being recirculated so that an 800 metre ponded sector
of the river is always maintained at an elevated temperature. It
is in this static zone that guppies have been found. During 1966
and 1967 weekly temperature readings throughout the year
showed that the river water always had a temperature within a
range between 16° and 30°C. and during June-August period no
figure below 25°C. was recorded.
The chemical quality of the river water is constantly checked
by staff of the Protection of Water Department of the Lee Con-
servancy Catchment Board and Table 1 shews the latest published
analyses of river water at Hackney. The poor quality of the River
Lee, as revealed by Table 1, is considered by Lee Conservancy
Catchment Board (1967) to be due to the discharge from Pymmes
Brook, consisting almost entirely of 40 million gallons per day
of sewage effluent, that enters about 2 kilometres upstream of
Hackney at Tottenham Hale (Grid Ref.: TQ 347890). Since (a)
the daily flow down the River Lee upstream of the confluence with