Physiography of Flowing Water - Freshwater Ecology: Concepts and Environmental Applications - page 70

surface into the stream (Europeans use the word catchment ). Watershed in
European terminology is the boundary of the catchment (i.e., the ridge that
divides catchments). Even though the ridge “sheds” water and the basin
“catches” it, I use the American definition of watershed.
Globally, more permanent streams and rivers occur in regions where
there is more precipitation (Fig. 5.2A). A greater number of intermittent
streams are found in drier regions (Fig. 5.2B). The discharge, or amount
of water produced by a stream per unit time, is approximately related to
the area of the watershed (Fig. 5.3). The amount of discharge produced per
unit area has an approximate upper bound. Many drier areas have lower
discharge per unit area than wetter areas, creating a 100-fold variance in
the relationship between watershed size and discharge for a watershed of
any given size. The world's largest rivers occur in large drainage basins
with significant amounts of precipitation. A list of the 15 largest rivers in
the world is presented in Table 5.1. The Nile is the longest river at 6758
km, but it only ranks 36th in discharge.
Stream order is another way to characterize streams. The most com-
mon method for ordering streams is the Strahler classification system, often
FIGURE 5.2 Distribution of counts (blue line features) of permanent (A) and intermittent
(B) rivers on Earth. Light areas are those with higher counts of rivers (Cogley, 1994).