Drainage basin

Ideally the historical record of stream water quaUty would extend back to a time when human activities in the drainage basin had no significant effects. This "pristine" condition had probably already passed in most U.S. rivers before any organized water quaUty studies were made, as concern about apparent stream pollution was commonly a motivating factor in starting such studies (see Water, pollution). 

This reaction is relatively fast and readily reversible so that in drainage basins in carbonate-dominated terranes the stream water commonly will have near-equihbrium concentrations of hydrogen, bicarbonate, and calcium ions. At equiUbrium, the rates of forward and reverse processes represented in equation 5 are equal. 

Selected analytical and related data for the example drainage basia are givea ia Tables 1 and 2. Apparent trends with time are evaluated by various means described ia the text. The aimual minimum and average constituent concentrations and aimual yields of sulfate in tons per unit area of drainage basin are the focus of the discussion. 

The influence of human activities in a stream drainage basin can be relatively simple and direct, as in the disposal of soluble organic and inorganic waste, or more subtie and complex, as in the conversion of prairie or forest land to agricultural use. Such effects can be expected to increase as population density and agricultural, industrial, and mining activities increase. 

The unique physical characteristics of the Great Lakes also contribute to their sensitivity to toxic substance inputs. The vast surface areas of the lakes makes atmospheric contributions of these chemicals, even at low concentrations, quantitatively significant. The relatively large ratio of the lake surface to drainage basin area, and the fact that the boundary of the drainage basin in some locations... 

Figure 4. Discharge of suspended sediment from world rivers in 10 tons ear. Sediment yield in tons/km ear for major drainage basins is shown by pattern (see key).

Watersheds, also known as drainage basins, define a natural context for the study of relationships among soils, geology, terrestrial ecosystems, and the hydrologic system because water and sediment travel downslope under the influence of gravity. This material is a continuation of some of what was presented in Chapter 6. 

The sediment load of a channel and the sediment yield of its drainage basin are expressed in different ways. The sediment load is the total mass of material moved by the river in a... 

The travel time for suspended load is controlled by the flow velocity and the distance to the basin outlet. Flow velocities do not change much downstream in a typical river system (Leopold, 1953) and typically range from 0.1 to several m/s. Hence, suspended load should be able to travel at least 10 to 100 km per day and the travel time for suspended sediment to traverse even the longest rivers in the world should be less than a season. Although some of the suspended load will be deposited in floodplains, the component of the suspended load that does not get sequestered in terrestrial depositional environments is delivered almost as fast as the water that it flows in. Bedload travels much more slowly. In mountain drainage basins, the velocity of individual bedload clasts is on the... 

Fig. 8-10 Erosion rate as a function of relief for (a) mid-latitude medium-sized drainage basins (modified from Ahnert, 1970), and (b) for Hydrographer s Volcano, Papua New Guinea (data source Ruxton and McDougall, 1967).

Ahnert, F. (1970). Functional relationships between denudation, relief, and uplift in large mid-latitude drainage basins. Am. /. Sci. 268, 243-263. 

Horton, R. E. (1945) Erosional development of streams and their drainage basins Hydrophysical approach to quantitative morphology. Bull. Geol. Soc. Am. 56,275-370. 

Drever, J. I. and Hurcomb, D. R. (1986). Neutralization of atmospheric acidity by chemical weathering in an alpine drainage basin in the North Cascade Mountains. Geology 14,221-224. 

Johnsson, M. J., Stallard, R. F., and Lundberg, N. (1991). Controls on the composition of fluvial sands from a tropical weathering environment Sands of the Orinoco River drainage basin, Venezuela and Colombia. Geol. Soc. Am. Bull. 103,1622-1647. 

Pinay G, O Keefe T, Edwards R et al (2003) Potential denitrification activity in the landscape of a western Alaska drainage basin. Ecosystems 6 336-343... 

Viers J, Dupre B, Polve M, Schott J, Dandurand L, Braun JJ (1997) Chemical weathering in the drainage basin of a tropical watershed (Nsimi-Zoetele site, Cameroon) comparison between organic-poor and organic-rich waters. Chem Geol 140 181-206... 

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