INLAND WATER SURVEY 411 
water supplies, land drainage, fisheries, hydro-electric schemes, electricity 
supply stations and various manufacturing processes, but also in attempts 
to control and prevent water pollution. The extent to which sewage or 
other polluting effluent ought to be purified is dependent partly on the 
diliition afforded by the river into which the effluent is discharged ; the 
greater the dilution the more rapidly the river recovers asa result of self- 
purification processes. A knowledge of the flow of the river or stream 
concerned is therefore necessary if plant for the treatment of sewage and 
industrial effluents i is to be designed and operated on the most economical 
lines. 
“Attention has been directed on many occasions to the importance of 
river flow data in’ relation to water pollution problems. The Royal 
Commission on Sewage Disposal, whose comprehensive inquiry during 
the period 1898 to 1915 dealt with the methods of treatment and disposal 
not only of sewage but also of trade effluents, definitely stated that the 
standards to be applied to sewage effluents should be adjusted according 
to the character of the streams into which they are to be discharged. ‘The 
same principle undoubtedly applies to trade effluents. During the inquiry 
a considerable number of rivers and streams of different. types were kept 
under observation with a view to tracing the effects of discharging various 
séwage liquids of known composition and volume into streams of known 
any. volume and velocity; and the scheme of standards which the 
oyal Commission finally recommended for sewage liquids was based on 
the results of these observations. The value of river flow data in dealing 
with problems of pollution has also been stressed in each of the published 
annual reports of the Water Pollution Research Board of the Department 
of Scientific and Industrial Research since the appointment of the Board 
in 1927. In addition, references to this same application of measurements 
of river flow have been made in published reports issued by the Ministry 
of Health, the Ministry of Agriculture and Fisheries and the West Riding 
of Yorkshire Rivers Board, and in other publications. 
’ Unfortunately, comparatively little information on the flows of rivers 
and streams in this country is available, with the result that in most instances 
the effects of discharges of polluting ‘effluents on individual rivers cannot 
properly be assessed. As an example of the dearth of information of this 
kind, even for important rivers, reference might be made to the river 
Tees. Early in 1929, the Water Pollution Research Board began a compre- 
hensive scientific survey of this river, with the object of obtaining reliable 
data regarding the quantities of polluting effluents of various kinds which 
can be allowed to enter a river without unduly retarding the natural processes 
of self-purification of the polluted river water. In this instance there 
were at the outset no gauging stations on the main river or its tributaries, 
and. no reliable records of river flow and levels. The chemical and 
biological work of the survey had to be supplemented, therefore, by hydro- 
graphical measurements not merely of tidal currents in the estuary, but also 
of freshwater flows in the upper river. 
In recent years the various authorities, undertakings and individuals 
responsible for dealing with pollution problems have undoubtedly begun 
to appréciate the importance of river flow data. A few have already made 
observations of the flows and levels of the rivers in which they are interested; 
and there are definite signs that, with encouragement, many others will be 
willing to arrange for river flow ‘and level measurements to be made. 
It may be asked whether the labour and expense likely to be involved 
in securing data on river flow can be justified in relation to the value of 
such data in dealing with pollution problems. In the first place river flow 
