and 1982. The situation in Essex is certainly no less alarming, and at the present time
Rainham Marsh is under threat of industrial development
2. Moving-Water Habitats
Some species, such as Pyrrhosoma nymphula and Ischnura elegans seem equally 'at
home' in still or moving water. Others, such as Lestes sponsa are mainly still-water
species which sometimes do occur in slow-moving rivers or canals. But another range of
species is largely or wholly confined to moving-water habitats. In one case (so far as the
Essex fauna is concerned) breeding sites are more-or-less exclusively swift-flowing
streams or reaches of rivers. Table 3:2 summarises present knowledge of the dragonflies
occurring in or near the main Essex river systems.
We can loosely distinguish three main types of waterway in Essex'. (a) canals, (b) slow-
moving (reaches of) rivers, and (c) swifter streams and (reaches of) rivers.
(a) Canals
Canals, in the sense of waterways wholly artificially created for such purposes as
transport and irrigation, are virtually absent from Essex except on the extreme western
metropolitan fringe. Where such waterways have artificial banks, and little or no
marginal vegetation, they support a small though still sometimes significant Odonata
population. The very local Erythromma najas, for example, breeds on one such stretch of
canal near Enfield Lock. It is more dependent on floating-leaved vegetation (favoured
by the minimal water-flow of canals) than emergent and marginal plants. The other
Essex canals (the Lee/Stort and Chelmer/Blackwater Navigations) are really 'canalised'
rivers, and maintain many of the ecological features of high quality river habitats. For
this reason they are included for purposes of this survey under the next heading.
(b) Slow-moving Rivers
Most of the Essex rivers rise in the relatively high ground of the north-west and have
their outflow either along the Essex coast or into the Thames. One exception is the Cam.
which rises in the north-west but flows north into Cambridgeshire. The Beam, the
Ingrebourne and the Mardyke have their sources in south-west Essex and flow into the
Thames, whilst the Crouch, the Roach, the Roman River and the Holland Brook all
have their sources in 'lowland' south or east Essex and flow into the sea.
The main river-systems which originate in the north-west (the Stour. Colne.
Blackwater, Chelmer, Roding and Stort/Lee) are generally narrow and swift-flowing in
their upper-reaches. Since this part of the county is under intense arable cultivation,
surrounding terrestrial habitat for adult dragonflies is usually minimal, marginal
vegetation is given little opportunity to develop, and there is considerable evidence of
problems caused by run-off of agricultural chemicals. Drainage in this area is also very
rapid, so that some channels run dry in the summer months. Such conditions are
uncongenial to most Odonata species, and the dragonfly fauna of the upper reaches and
tributaries of most of these rivers in the north-west is poor to non-existent. Some, such as
the Cam. support such species as Ischnura elegans and Aeshna grandis. The western
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