An Environmental History of Wildlife in England 1650-1950

(^84) an environmenTal hisTory of Wildlife in england
it was ‘ regarded as a “ mecca ” amongst botanists and entomologists ’. 48 It
currently supports 102 different species of aquatic plant, more than any
natural river in England, 44 of which are rare at a national or regional level.
This diversity results in part from the fact that the canal cuts through a
range of geological formations, and is thus fed by water sources which are
more varied, in terms of their PH especially, than those associated with the
tributaries of a natural river.
Where canals crossed watersheds reservoirs needed to be constructed, for
some water flowed downhill each time locks were opened. Reservoirs were
also needed more generally to compensate for loss of water through leakage
and evaporation. These have often developed as important wildlife reserves;
now over two centuries old, they have generally lost any artificial appearance
and are usually surrounded by reed beds. The four constructed near Tring
on the Buckinghamshire/Hertfordshire border to supply the Grand Union
(then the Grand Junction) canal have long been one of the most important
sites in England for wintering water birds or passing migrants, especially
terns and various waders. One of the four, Wilstone reservoir, is famous
figure 17 Above: navigable waterways in England and Wales in (a) 1760 and
(b) 1850. Below: the rail network in England and Wales in (c) 1840 (d) 1850
(c) 1922 (after Cossons 1987).