it enjoyed such inland tidal conditions; and of the Thames
from Greenwich to Teddington that similar advantages existed.

The true point of division which separates, so far as human history is
concerned, the lower from the upper part of such rivers is the first
bridge, and, what almost always accompanies the first bridge, the
first great town. To repeat the obvious parallel, Rouen was this point
upon the Seine; upon the Thames this point was the Bridge of London.
It is with the habitable and historic Thames Valley above the bridge
that this book has to deal, and it will later be to the reader's
purpose to consider why London Bridge crossed the stream just where it
did, and of what moment that site has been in the history of the
Thames and of England.

The second division in a great European tidal river, considered as a
means of communication, is the navigable but non-tidal portion.

The word navigable is so vague that it requires some definition before
we can apply it to any particular stream. It does not, of course, mean
in this connection "navigable by sea-going boats." One may take a
constant depth of so little as three feet to be sufficient for the
purpose of carrying merchandise even in considerable bulk.

The legislatures of various countries have established varying gauges
to determine where the navigability of a river may be said to cease.
In practice these gauges have always been arbitrary. The upper reaches
of a river may present sufficient depth but too fast a current, or
they may be too narrow, or the curves may be too rapid, or the
obstruction of rocks too common, for any sort of navigation, although
the depth of water be sufficient.

Conversely, in some streams of peculiar breadth and constancy very
shallow upper reaches may have early been converted to the use of man.
The matter is only to be determined by the experience of what the
inhabitants of a river valley have actually been able to do under the
local circumstances, and when we examine this we shall usually be
astonished to see how far inland a river was used until the history of
internal navigation was transformed by the development of canals or
partially destroyed by the development of railways. Thus it is certain
that so small a stream as the Adur in Sussex floated barges up to the
boundaries of Shipley Parish; that the Stour was habitually used
beyond Canterbury; that so tiny a tributary as the Ant in Norfolk was
followed up from its parent