INTRODUCTION

The volume and quality of water accumulating in natural lakes and wetlands varies tremendously from place to place on the continents and over time (Figures 7.1 and 7.2). In most parts of the modern world wetlands have been subject to drastic changes caused by water and land management practices, and lakes generally have received much more attention than wetlands. The extent and types of change in both quantity and quality of water are influenced by a diverse combination of human techniques that are evolving rapidly (Figure 7.3).

The current situation is reviewed here in terms of the major types of lakes and wetlands in the effects of human management, ranging from virtual destruction of large areas, as in the case of sections of the Aral Sea, to the preservation of unique landscapes, as in the case of small lakes in the Adirondacks and Japan.

In some regions, such as in southern Florida, the distinction between gently sloping perennial wetlands and shallow lakes may be slight, and certain lakes are bordered by seasonal wetlands. In some other areas such as Lake Baikal, the boundaries between deep lake bottom and rugged shores are abrupt and well defined.

It is estimated that the earth's continents contain roughly 191 million km2 of liquid surface water, and that 92.3 percent is in lakes is that in volume they account for approximately five percent of the lakes, but because they extend seasonally over large shallow areas, the total area of wetlands on continents in 2000 was highly variable and extremely difficult to estimate.