252 THE ESSEX NATURALIST
could lead to widely varying results in qualitative or quantitative
estimations by two biologists sampling with the same collecting
methods in a stretch of river.
It is of the utmost importance that the physical conditions of
the various riffles be similar. The most important is that Dry
Weather Flow (D.W.F.) conditions should exist. This may be
defined in a number of ways, but the definition I have adopted as
most suitable is that D.W.F. conditions exist after a period of
fourteen consecutive days with no more than 0.02" of rain on any
one day.
It must be remembered that pollution is most serious in the
summer months when D.W.F. conditions exist. Pollution does
occur in winter months but the incidence is not as high. Also, in
winter months the flora and fauna of a river bed are sparse and
therefore very difficult to estimate and, moreover, D.W.F. con-
ditions rarely exist.
The rate of flow over a riffle in rainy weather restricts the
method of sampling and the physical state of the riffle is very
difficult to define. Indeed the population density of various
genera of the riffle community might well become depleted by
organisms being swept away in the excessive river flow.
It has been found that fourteen days is sufficient time for the
riffle habitat to become re-established, except after storm flow
conditions. Dry weather flow conditions define the rate and
volume of flow existing in various rivers. In four Essex rivers the
average Dry Weather Flow is as follows:
Million
gallons/day Cu. ft. /secs.
River Stour at Langham 10.2 19
River Colne at Lexden 3.2 6
River Blackwater at Langford 4.9 9
River Chelmer at Rushes Lock 3.8 7
I consider that there is no justification for a quantitative estima-
tion of aquatic organisms since there may be such a large
variation in their distribution. Therefore I have chosen to
estimate them qualitatively in so far that their frequency of
occurrence is noted.
The symbols to be used in this paper are : —
rare = r; frequent=f; very frequent = v.f.; abundant = a.
The samples obtained should be as representative as possible.
This being so, one should take the precaution of disturbing the
riffle as little as possible. It makes this easier if, before sampling,
a rough sketch is made, taking particular note of the type of
aquatic vegetation, both upstream and downstream, and those
points on the riffle most likely to yield information about the area
being assessed as in Fig. 3.