We can infer from the passage that today ………….

In mediaeval times rivers were the veins of the body
politic 'as well as economic. Boundaries between
states or shires ,they were crossed by fords which
became the sites of towns, or by bridges which were
often points of battle. Upon rivers the people of that
time depended for food, power and transport. In our
day fish are caught in the sea and brought to us by rail
and lorry; only the angler still thinks fresh-water fish
important, and pollution of rivers drives him into smaller
and smaller reaches in which to practise his sport.