Geoscience Reference
In-Depth Information
ing lower general rainfall and different groundwater conditions. The gauging stations
on the south and east side of the Cotswold drainage divide are the sources and upper
reaches of the Nene, Great Ouse, Cherwell and Windrush, with catchment areas in Area
9 of no more than a few hundred square kilometres, and mean river flows of no more
than 4 m
3
/s.
Drainage to the Severn Estuary
As described above, the Severn and Avon river systems have by far the largest catch-
ments and river flows in Area 9. Where the River Severn flows into Area 9, it has gen-
erally incised a few tens of metres into a landscape underlain by New Red Sandstone.
A
distinct floodplain, typically half a kilometre in width, has been built by river sedi-
mentation during the Holocene (postglacial) rise of sea level, but there are often small
patches of river terraces at higher levels representing earlier floodplains. Along the rest
of its downstream course across Area 9, the Severn continues to display similar fea-
tures (Fig. 175). However, increases in the width of the young floodplain, the extent
of ancient river terraces, and the size of meander bends all correspond to the steady
increase in mean flow downstream as the catchment area and number of tributaries in-
crease.
The River Avon is the largest tributary of the Severn, joining it near Tewkesbury
after flowing parallel to the Cotswold drainage divide. The tributaries of the Avon that
join it from the southeast have been the main agent in eroding the northwestern slopes
of the Cotswolds (Fig. 174). The River Avon itself is incised into landscapes made
largely of impermeable mudstones of Triassic and Early Jurassic age (Fig. 176). Its
young floodplain is locally more than 2 km across, and it has particularly well-deve-
loped large meanders (several kilometres in wavelength) in the reach just above and
below Evesham. The meanders become smaller - though they are still well developed
- in the reach upstream towards Stratford-on-Avon, as the mean flow becomes less. In
the area between Coventry and Rugby, large areas of the valley slopes are occupied
by surface-blanket deposits, particularly ice-laid materials and water laid muds, sands
and gravels. The most important of these, from a landscape point of view, are the Dun-
smore Gravels, which form a distinct plateau southeast of the Avon, between Stratford-
upon-Avon and Leamington Spa. Here they provide a clear scarp to the river Leam, a
southern tributary of the Avon. The age of these gravels is not clear, but they represent
a major episode of river deposition that was probably linked to one of the Ice Age cold
phases. Whether this was the Anglian cold episode, about 450,000 years ago (oxygen
isotope stage 12), or one of the later cold episodes is still uncertain.
