Abstract The waters of the Earth circulate from the oceans through the atmosphere to the land, from which a part is returned to the atmosphere and the residual is returned to the ocean either by streams or underground flow. The paths of water on the land include surface flow to stream channels, infiltration to the soil, movement through the ground to a ground-water body and thence to a stream channel, and eventual return to the ocean. Water is extracted and returned t the atmosphere by evapotranspiration (evaporation and transpiration) throughout much of the land phase.

Abstract A tenet of geomorphology and hydrology holds that rivers are ultimately the products of processes occurring on the landscape. Geology, topography, and climate reflected in rainfall, runoff, and vegetation determine the characteristics of the flow of water in the river channel as well as the quantities and characteristics of clastic and dissolved materials transported by rivers. Where river channels are unimpeded by bedrock and are truly alluvial—capable of adjusting to changes in flow and sediment input, and flowing within materials transported by the river itself—it has been demonstrated that rivers adjust to the flows of water and sediment derived from the watershed. At the same time, geologists recognize that many river reaches reflect history and local influences as well. These essays on the riverscape attempt to capture these diverse hydrologic and other influences determining the look of a river at a given place. The riverscape here means the river itself and its immediate surroundings. The riverscapes described are ones for which good information is available. They demonstrate a variety of influences and conditions under which particular ones dominate the scene. Each, of course, could warrant a book, and these vignettes are not intended to be comprehensive, but rather illustrative. (The editors, not the authors, are responsible for the constrained space.) The rivers chosen have distinct qualities. The Nelson River, for example, is one of the large rivers of the world; it flows northward from proximate sources in Lake Winnipeg, but its sources are also in the Rocky Mountains, draining to the Saskatchewan River.

Abstarct Streamflow occurs as a result of the interaction of the many components of the hydrologic cycle, as shown in Figure 5 of Riggs and Wolman (this volume). The streamflow part of the cycle is driven by precipitation in the form of rain or snow. The complex processes of interception, infiltration, evaporation, and transpiration (Saxton and Shiau, this volume) serve to reduce the amount of water available for runoff at any one time and at any one location. These proceses are influenced by various climatic factors, such as precipitation, temperature, wind, and solar radiation; and by physiographic factors related to geology and topography. As a result, streamflow varies considerably both in time and space. For example, see the variation of daily streamflows in Figure 8 of Riggs and Wolman (this volume). Lins and others (this volume) and Saxton and Shiau (this volume) describe the climatic and physiographic variations throughout the continent. This chapter describes the variation of runoff (the volume of flow for a given time) in North America and explains the major climatic and physiographic factors underlying this variation. Following a description of common terminology related to runoff, the variation of runoff in various parts of the continent is presented on maps and graphs. The next section describes the effects of climatic, topographic, and geologic factors on runoff using appropriate examples. A final section documents the integrated effects of climate and physiography within a specific watershed as runoff drains from the headwaters to the basin outlet.

Abstract Surface water hydrologie processes represent the dynamic expression of the flux of moisture to, across, and from the land surface. The primary control on this moisture flux is climate, which in turn, is controlled by the general circulation of the atmosphere. Thus, in order to understand the nature and characteristics of surface water hydrology, it is first necessary to understand the atmospheric context within which surface water processes occur. In this chapter we present this context in a conceptually broad, but topically systematic way. The chapter content includes empirical information developed over many decades of observation. .We develop a more contemporary perspective, however, by characterizing these observations in a framework that draws on the current thinking associated with global change, especially with respect to climate variability and change. The chapter begins with an overview of the concepts and elements associated with the fluxes of energy and moisture to, across, through, and from the land surface. In particular, emphasis is placed on the characteristics of the surface energy balance, especially the fluxes of latent and sensible heat. Additionally, the primary terrestrial components of the hydrologie cycle— évapotranspiration, runoff, and soil moisture—are discussed. From this description of the atmospheric “engine” we proceed to a characterization of the climate and hydrologie effects that result from the operation of the atmospheric “engine.” This includes the general atmospheric circulation (i.e., the weather delivery system), the climatology of North American extratropical and tropical storms, patterns and trends in North American droughts, regional hydroclimatic conditions, and an overview of the hydrologie cycle over North America.

Abstract 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.

Abstract The term “floods” can have quite different connotations to hydrologists and to laymen. Hydrologists are primarily concerned with discharge, the volume per unit of time at a point, along a stream, or throughout a drainage basin or basins, usually on a yearly basis. However, in arid regions the “annual flood” in some years may be a very small discharge or even zero-certainly not a “flood” in the lay sense. Even on a perennial stream, the annual maximum discharge or “flood” during a drought year may be a flow that is exceeded on many days in a wet year.

Abstract Streamflow fluctuates in time in response to the annual cycles of precipitation and temperature and to random deviations from these cycles. The resulting lowest flow for each year, usually an average for some number of consecutive days, is designated as an annual low flow. That flow usually occurs during a period of no precipitation and is derived from ground-water discharge or surface storage being discharged from lakes, marshes, or melting glaciers.

Abstract The surface-water components of the hydrologic cycle usually are considered to be rainfall and subsequent runoff. Yet, in many parts of North America, for a major part of the year, the more appropriate concept is precipitation and storage as snow and ice, followed by melting, followed by runoff. The timing involved in these two concepts is very different: Runoff follows rainfall almost immediately, whereas the time between snowfall and melt can range from days to months for seasonal snowcovers, and from months to millennia for glaciers. The predictive analysis of runoff also is very different: Prediction of runoff from rainfall requires knowledge of the precipitation pattern in time and space, whereas the prediction of runoff from snow and ice melt requires knowledge of the amount and distribution of snow in storage (measurable), and the meteorologic conditions that cause melt. The fact that snow and ice accumulate and melt to produce runoff in a very different way than does rainfall commonly is ignored in a simple hydrologic analysis.

Abstract The existence of lakes and wetlands depends on the specific geologic setting that favors the ponding of water, and on the hydrologic processes that allow the body of water to persist at a given site. Lakes can occur only in topographic depressions, but wetlands occur in depressions, on flat areas, on slopes, and even on drainage divides. Lakes and wetlands have some common characteristics, but they differ in many aspects of water storage, water circulation, water loss to the atmosphere, and the thermal and chemical characteristics of their waters.

Abstract This chapter has three principal objectives: (1) to summarize the present chemical composition of North American surface waters and point out any discernible trends with time; (2) to review chemical and biochemical principles and processes that control natural water composition, and the ways in which these may be involved in attaining the particular chemical compositions and trends that we can observe; and (3) to point out some of the more important factors that must be considered in collecting surface-water-quality data. This discussion is concerned principally with inorganic chemistry and geochemistry. However, biochemical processes in river and lake water influence their chemical composition extensively, and specific effects are pointed out. Aquatic biology is discussed in another chapter. The physical processes occurring in lakes also have very important effects on water chemistry; these aspects of limnology are covered more fully in a separate chapter. Data on which this chapter is based relate mostly to waters of Canada, the United States, and Mexico. For most of Central America, very little water-quality information is available. Tables 1 through 3 contain analytical data for river waters that illustrate some of the principles discussed, and summarize major features of the chemical composition of North American surface waters. To a certain extent, at least, one would expect the average concentrations of dissolved elements in surface fresh water to reflect the relative abundance of the elements in the crustal rocks exposed at and near the land surface.

Abstract This chapter consists of two sections that represent complementary approaches to the description of the distribution of aquatic biota on the North American continent. The first section, by Patrick, deals with aquatic life in rivers in the United States and emphasizes the geologic, hydrologic, and hydraulic characteristics that affect the spatial distribution of the biota. These principles, as indicated in the section by Williams, also relate to distribution throughout the North American continent. The geographic distribution itself is elucidated in the text and is organized around the factors influencing that distribution. The second section, on the distribution of biota in Canadian and other North American waters, describes the spatial or geographic distribution of major aquatic biota in the river systems of Canada. It includes maps depicting the geographic distribution throughout the North American continent of selected species of fish, indicative of the wide range of spatial characteristics of these distributions. Together the two sections emphasize both the geographic distribution throughout the North American continent of selected species of fish, indicative of the wide range of spatial characteristics of these distributions, and the factors influencing those distributions throughout the continent.

Abstract Sediment in river systems is of interest to earth and water scientists working at problems that span several time scales. On the longest time scale (10 8 to 10 9 years), sediment is the major form in which material is transferred from continents to oceans, and the rates at which river sediment has been produced in the geologic past are major concerns of those who study longterm geochemical cycling. At somewhat shorter time scales (10 6 to 10 8 years), the properties of the existing sedimentary rocks and deposits are the major clues to past hydrologic and geologic conditions, and, to the extent that the present really is the key to the past, it behooves us to understand how today’s observable conditions influence today’s river sediments. On a more secular time scale, sediment in rivers is of immediate concern as a reflection of soil erosion, as a major design consideration for reservoir sedimentation, river navigation, and other engineering works, as a transporter of various materials (toxic and otherwise) that are adsorbed onto sediment particles in river systems, and as an influence on the habitat of aquatic wildlife. These secular-scale considerations have prompted research and monitoring activities during the past half century that have provided much of our basic knowledge of sediment in the river systems of North America. Studies of soil erosion and valley sedimentation became especially intensive and extensive during the 1930s; these studies continue today, mostly under the aegis of the national agricultural agencies of the United States and Canada.