(c) Swiftly-flowing Rivers and Streams
Though there is no clear dividing-line between this category of sites and the last, it is
obvious that rate of flow of water makes a significant difference to the aquatic
environment of dragonfly nymphs. The amount and nature of aquatic vegetation will
differ, as well as the nature of the river-bed. A muddy, 'depositional' bed is typical of
slow-moving reaches, whilst a stone or gravel-covered river-bed is characteristic of swift,
'eroding' reaches. Two rivers in the north-east of the county, the Roman River and the
Holland Brook exhibit 'riffle and pool' characteristics for a considerable part of their
length, as do the larger rivers of Essex in their upper reaches.
The poverty of the Odonata-fauna of the upper reaches of those rivers which rise in
the north-west has already been mentioned. The Holland Brook suffers some pollution
from agricultural run-off and is tidal in its lower reaches, two factors which limit its
suitability for dragonflies. However, it does sustain two species which appear to be
adapted for life in more rapidly moving aquatic habitats. One of these is the ubiquitous
Ischnura elegans and the other is the more local Pyrrhosoma nymphula. The Roman
River, which runs east to west just south of Colchester, has benefited from sympathetic
management owing to its situation within a conservation zone (Wake. 1984). The swifter
reaches of the Roman River and one of its tributaries support both of the above-
mentioned species, together with Calopteryx virgo (the Beautiful Demoiselle). The
Roman River is probably the only breeding site for this species in Essex, and it is also
one of its most easterly outposts in Britain. The species is mainly western in its British
distribution, and is said (Corbet. Longfield and Moore. 1985. p.45)to favour more acidic
waters than its close relative C. splendens. Possibly this latter feature, together with
problems of agricultural pollution, could explain the apparent absence of C. virgo from
visually suitable waterways in the north-west of the county.
The management of the Essex rivers is almost wholly within the control of the Water
Authorities (the Thames, for those which flow out into the Thames Estuary, and the
Anglian for those which flow directly into the sea). Larger scale drainage and flood-
prevention schemes, often involving straightening of the river-course, deepening and
increasing the rate of flow, are a direct threat to the quality of the aquatic habitat of
dragonflies, but they also have the indirect consequence of enabling surrounding areas
of marsh or water meadow to be ploughed up for arable agriculture. This means a
further risk to the aquatic environment from fertiliser run-off and spray-drift, and the
loss of adjacent feeding-sites and shelter for adult dragonflies. Dredging operations and
mechanical clearance of marginal and bankside vegetation, the outflow of sewage and
industrial pollutants into rivers, and excessive interference from incompatible
recreational uses of the rivers can also seriously affect dragonfly populations.
Such rivers as the Lee. the Chelmer/Blackwater Navigation and the Roman River
have benefited from a measure of protection from adverse commercial developments
because their high conservation value has been generally recognised, and because local
resistance has been aroused when necessary. It is not surprising to find that these three
rivers have a very rich and important dragonfly fauna. The Stour, which forms the
northern boundary of Essex for much of its length, provides an interesting point of
comparison. Its water quality is rated as high (Corke. 1984. p. 117ff). and it seems to have
formerly supported a rich Odonata fauna, including several rare or local species. It now
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