The influence of man on hydrologic systems
No discussion of the surface-water resources of North America would be complete without explicit consideration of the role that man plays in determining the flow of water through river systems. Human activities that may affect flows include diversion of water from one river basin to another, creation of artificial reservoir storage, destruction of natural wetland storage, and land changes that alter rates of erosion, infiltration, overland flow, or evapotranspiration. The effects of these human actions influence not only long-term average flows, but the magnitude and frequency of droughts and floods and year-to-year and season-toseason flow variations. These effects, in turn, have a variety of direct effects on man, related to availability of reliable water supplies for in-stream and withdrawal uses and to magnitude and frequency of flood damages. They also affect geomorphic features resulting from changes in the temporal distribution of erosive or transporting forces. The result of these are changes in the geometry and sedimentary characteristics of river channels, flood plains, and deltas.
Types of activities that affect river flows are illustrated in this chapter by using a number of examples. The extent and nature of the origins of the change are described where possible, and examples of their effects are given. Flood flows are emphasized because of their particularly great geomorphic significance in determining channel geometry and sediment characteristics. In addition to consideration of flood flows, the flow-duration curve (frequency distribution), patterns of seasonality, and extent of year-to-year variability are discussed.