Rivers and Lakes [antikvár]

Preface A major editorial problem encountered in preparing this encyclopedia has been to decide which of the world's many thousands of rivers and lakes to describe in detail. Size is an obvious criterion, but there are often matters of particular interest and importance unconnected with size, so that the ultimate selection for this book has been partly subjective. To supplement nearly 500 main articles, however, we have a gazetteer with more than 2,000 brief entries, and an index. There are further problems, of course, in defining a river. Does it start where its name is first used, or at the source of its main headstream? Does it end where tidal effects are first felt, where the water begins to be brackish, where the banks fali away in the estuary, or not until open sea has indisputably been reached? Does the Thames, for instance, end at Tilbury or beyond Southend? The Susquehanna at the head of Chesapeake Bay or at its mouth? (The Bay, of course, is only the drowned lower course of the river - a further complication.) When stating a length, should this be a measurement down the middle of the valley, or round each meander? Sometimes no survey has given an exact figure on either basis, and sometimes the length is in dispute. Each case has been decided on its merits, for outlets vary in character and do not conform to clear and uniform rules, while the continuity of a river-system with its headstreams is alsó not always clear. These are somé of the reasons why the lengths of rivers are often given in this book as approximate. Figures for the dimensions and area of lakes are alsó approximate, in many cases. Lakes are never rectangular, and are often very irregular in shape, with many arms and bays: sometimes it is difficult to be sure whether there is just one laké or a group of lakes joined by channels. We have not included canals, although the canalization of rivers has of course been referred to, since canals are artificial constructions whose interest is chiefly a matter of engineering and transport - two topics that this book is not primarily concerned with, though it touches upon them. For similar reasons, artificial lakes förmed by reservoirs have been omitted, with one or two exceptions. A stretch of water was not considered to be a laké if the water contained salt from the open sea, and so coastal lagoons have generally been omitted. So have fjords, which are salt-water inlets. To give a summary of the geography of rivers in generál, their formation and behavior, the problems they create and the uses to which they can be put, we have included an introductory article entitled "River and Lake." In all, we have aimed to convey something of the humán interest of the world's rivers and lakes, as weil as of their physical geography. We hope this volume will encourage further study of one of the major features of man's environment. R. Kay Gresswell Anthony Huxley