Geoscience Reference
In-Depth Information
Over millions of years, river down-cutting, slope erosion and material transport
tend to smooth and lower landscapes until they approximate plains, unless they are
raised up again (
rejuvenated
) by large-scale Earth movements (Chapter 3) or are at-
tacked by a new episode of channel erosion, perhaps due to climate or sea-level change.
Southern England generally has a smoothed and lowered landscape, representing hun-
dreds of thousands of years of this river and slope activity.
The branching, map-view patterns of river channels and valleys are an obvious
feature of all landscapes. An approach to understanding how this forms is illustrated
by a computer-based experiment (Fig. 12) in which a flat surface (plateau or plain) is
uplifted along one of its edges, so that it has a uniform slope towards the edge that
forms the bottom of the rectangle shown. Rain is then applied uniformly across the sur-
face, causing the formation and down-cutting of channels that erode backwards from
the downstream edge. As the experiment continues, the channels and their valleys ex-
tend into the uniform sloping surface by
headward
erosion, resulting in longer valleys,
more branches and a greater dissection of the surface by those valleys.
FIG
12.
Model showing upstream erosion by tree-like (dendritic) river patterns. (Provided
by Dimitri Lague from the work of A. Crave and P. Davy)
As we consider the various Regions and Areas of Southern England, we will sum-
marise the present-day river patterns of each by simplifying the main directions of
drainage involved. We will also give an impression of the present-day relative size of
the more important rivers by quoting their mean flow rates as estimated in the National
River Flow Archive, maintained by the Centre for Ecology and Hydrology at Walling-
ford.
It seems surprising that today's often sleepy southern English rivers have been
the dominant agent in carving the English landscape. However, even today's rivers can
become surprisingly violent in what are often described as hundred-or thousand-year
floods. Floods in the past were certainly more violent at times than those of today, par-
ticularly towards the ends of cold episodes, when melting of ice and snow frequently
produced floods that we would now regard as very exceptional.
