River conservation in the United States and
J.R. Karr, J.D. Allan and A.C. Benke
Rivers in Canada and the United States, like rivers world-wide, are shaped by their landscapes as much as landscapes are shaped by rivers. Both are defined by regional geology, topography, rainfall, temperature and living organisms. As a result of the complex interactions of climate, running water and land, rivers have been changing for millennia, but the rapid growth over the past 200 years of human populations and their technologies has been a new force for change, altering US and Canadian rivers radically from what they were 300 years ago.
The 24.2 million km2 of the United States and Canada (16% of the world’s total land area) extends from 25 to 70°N latitude. The continent is marked by major landscape features, such as the western and eastern mountain ranges (the Rocky Mountains and the Coast Range in the West, and the Appalachians in the East) and the Laurentian Great Lakes on the border between the two countries. This variation in physical environment has given rise to at least six major terrestrial biomes: tundra, coniferous forest, temperate deciduous forest, grassland, desert and chaparral. Consequently, the major rivers vary tremendously in drainage area, run-off volume, channel form and biotic attributes (Figure 1.1, Table 1.1). The watersheds of two major rivers (Colorado and Rio Grande) include Mexico, an area beyond the scope of this chapter. (Biogeographically, Mexico is part of North America but the focus in this chapter arbitrarily excludes Mexico.)
Total river run-off averages 8200 km3 yr‒1 or 17.6% of the world total (Shiklomanov, 1993; World Resources Institute: WRI, 1994). Approximately twothirds comes from the surface, one-third from groundwater. Large rivers such as the Mississippi, Colorado and Columbia run through several biomes (Table 1.1). The continent’s western border illustrates the extreme range of physical features and river characteristics: annual precipitation varies from less than 10 cm in the arid south-western US to 400–500 cm along the north-western coastal mountains (Fisher, 1995; Mackay, 1995; Oswood et al., 1995; Patrick, 1995). Precipitation in the central temperate grassland ranges from 20 cm in the Intermountain West to 100 cm in the East (Brown and Matthews, 1995; Patrick, 1995). In the temperate deciduous forests of the eastern United States and south-eastern Canada, precipitation varies from 70 cm in Wisconsin and Ontario to more than 200 cm in the southern Appalachian Mountains (Mackay, 1995; Patrick, 1995; Webster et al., 1995). Precipitation may exceed 100 cm in the Canadian Rocky Mountains but declines to 20–70 cm in coniferous forest and tundra biomes of Alaska and northern Canada (Mackay, 1995; Oswood et al., 1995).
Consider the contrast in climate-caused run-off between two western rivers. The extensive Colorado River basin (635 000 km2) in the arid Southwest (Figure 1.1) has a mean annual discharge of only 640 m3 s‒1 (Meybeck, 1988); precipitation is low and evaporation is high throughout. Present-day heavy withdrawals of water in the United States reduce the Colorado’s
Global Perspectives on River Conservation: Science. Policy and Practice.
Edited by P.J. Boon, B.R. Davies and G.E. Petts. © 2000 John Wiley & Sons Ltd.