Sun, wind and water draining from the land interact with the morphological features of a water body to create the environment experienced by freshwater plants and animals. The result of this interaction can be considered as the freshwater hydro climate and this plays the same role as that of conventional climate in terrestrial ecology. Agriculture, for example, has long been supported by specialist meteorological services which not only provide farmers with a sound interpretation of weather and climate without excessive technicality but which also consider relations between climate and the growth of crops and stock. There is a need for a similar service in freshwater ecology and applied biology. This book is the result of a number of years devoted to developing part of that service. It concentrates on the influence of all forms of water movement on the ecology of fresh waters. Water movement implies interest in both the quantity of water moving through river basins which reflects the climate of the catch­ ment as well as the nature of the fluid motion within the rivers and lakes of the basin. The book is not so much a review of recent research as an attempt to establish a logic-how knowledge of water movement can contribute to understanding the ecology of fresh waters. Two points follow directly.