Surface waters of North America; Influence of land and vegetation on streamflow
Surface waters originate as precipitation determined by local climate, but they must pass over or through the land surface before accumulating as streamflow. Climatic considerations were presented in the previous chapter. In this chapter, major factors that influence the transformation of precipitation into streamflow are presented.
Although precipitation patterns and intensities largely determine the surface runoff from a particular drainage basin, two drainage basins subjected to the same climate may have dissimilar runoff characteristics due to differences in topography, geology, soil and vegetation. Striking contrasts at once come to mind: Rugged mountain areas with their torrential streams, contrasted with aggrading deltas and sluggish waters; maturely dissected areas with their prompt and complete drainage, contrasted with areas of recent glacial drift in which lakes and swamps abound; well-forested lands that tend to hold the rain and snow, contrasted with denuded lands that allow quick runoff; areas of cavernous limestone with their extensive subterranean drainage and big springs, contrasted with areas of relative impermeable granitic rock that have well-developed systems of surface streams and numerous small springs. (Meinzer, 1942, p. 6)
Many examples of diverse hydrologic responses are shown in subsequent chapters. In this chapter the hydrologic processes that cause such responses are described and examined for major sections of the North American continent.
To consider surface waters on a continental scale, it is only possible to present a broad scale of features for this land phase of the hydrologic cycle. But large watershed land areas are the integration and interaction of many smaller watersheds. Thus, the hydrologic processes of unit land areas and small watersheds provide understanding for larger areas. Both broad-scale descriptions of the North American continent and more detailed hydrologic processes are useful to describe surface-water interactions with the land and vegetation.