Walker - 1967 - A geography of Italy

PART I I I : REGION A L GEOGRAPHY
They have been exploited for at least eight hundred years and even today they
water over one-third o f Lombardy’s irrigated area.
5 THE LOWER PL A IN ( b a s s a p i a n u r a ) . The northern margin o f this zone of
fine, only slightly permeable fluvio-glacial deposits lies at about loom in Lombardy
and at about 50m in Veneto; it shades off without an obvious break into
the floodplains o f the Po and its feeders. The water table is high and although
this part o f the plain lends itself to irrigation, its development must be preceded
by the estabUshment o f satisfactory drainage, especially where ferretto occurs, as
in the Vercellese.
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6 THE FLOODPLAINS OF THE PO AND ITS ALPIN E TRIBUTARIES. Both along the
Po and its major left-bank feeders the fall-away from the interfluvial platforms to
the modern floodplains is more marked upstream. The interfluves have been
reduced by lateral corrasion and although the process has been checked by
dyking, the floodplains have been repeatedly worked over by migrating meanders;
while these may now be drained and cultivated, the outlines o f abandoned
meanders (mortizze) are obvious from the air. The reclamation o f the tributary
floodplains, at least upstream, was a comparatively simple matter; part o f the
river’s flow was diverted along the foot o f the interfluve and allowed to drain back
by gravity through minor drainage and irrigation channels to the river bed. The
apphcation o f this method to the Po floodplain was ruled out by the scale of
operations required and by the imperceptible gradient. Below the junction with
the Oglio the difficulty o f drainage by gravity increases; except in summer the
water table o f the barely inchned recent alluvium, much o f it derived from the
Apennines, is almost at the surface and the movement o f water laterally is impeded
by slightly upraised fossil river beds and by lidi left from former shorelines.
From the sixteenth century onwards attempts were made to drain this
fenny zone by straightening and banking the rivers and by cutting a network of
gravity-flow channels. These efforts were frequently defeated by exceptional
floods and by the lowering o f the surface through shrinkage, but where they met
with success these areas are known as the terre vecchie. Elsewhere reclamation had
to await the application o f efficient pumping machinery in the later nineteenth
century, a development which has won large tracts o f Rovigo and Ferrara provinces
for cultivation. Unfortunately the history o f the Po fenlands is too
repetitively catastrophic to encourage rash prophecies as to the permanent
security o f the recent bonifiche. In Ravenna province a method o f reclamation
known as colmate naturali has been successfully employed; essentially it is the
diversion o f the heavily charged flood waters of the Apennine rivers on to an enclosed
area until the accumulation o f silt raises the surface to the required level.
7 THE COASTAL LAGOONS AND DELTAS. In a different context this area has already
received attention (p. 83).
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