Page 247 - Fundamentals of Geomorphology

230 PROCESS AND FORM

a
b
( ) Stream order (Strahler) ( ) Stream magnitude (Shreve)
1 1
1 1
2
4 18
3
1 2 1 2 15
14
4
2 2 13
1 1
1 1
1 4 1 1 11 1
1 1
2 3 2 5
1 3 1 1 6 1
2 1 3 1
2 3 2 4
2 2 2 2
1 1
1 2 1 1 2 1
1 1
1 1 1
1 1 1
Figure 9.5 Stream ordering. (a) Strahler’s system. (b) Shreve’s system.

a lower order. Strahler’s system takes no account of The main geometrical properties of stream networks
distance and all fourth-order basins are considered as and drainage basins are listed in Table 9.1. The most
similar. Shreve’s ordering system, on the other hand, important of these is probably drainage density, which is
defines the magnitude of a channel segment as the the average length of channel per unit area of drainage
total number of tributaries that feed it. Stream mag- basin. Drainage density is a measure of how frequently
nitude is closely related to the proportion of the total streams occur on the land surface. It reflects a balance
basin area contributing runoff, and so it provides a between erosive forces and the resistance of the ground
good estimate of relative stream discharge for small river surface, and is therefore related closely to climate, lithol-
systems. ogy, and vegetation. Drainage densities can range from
2
Strahler’s stream order has been applied to many river less than 5 km/km when slopes are gentle, rainfall low,
systems and it has been proved statistically to be related to and bedrock permeable (e.g. sandstones), to much larger
2
a number of drainage-basin morphometry elements. For values of more than 500 km/km in upland areas where
instance, the mean stream gradients of each order approx- rocks are impermeable, slopes are steep, and rainfall totals
imate an inverse geometric series, in which the first term are high (e.g. on unvegetated clay ‘badlands’ – Plate 9.5).
is the mean gradient of first-order streams. A commonly Climate is important in basins of very high drainage den-
used topological property is the bifurcation ratio, that is, sities in some semi-arid environments that seem to result
the ratio between the number of stream segments of one from the prevalence of surface runoff and the relative
order and the number of the next-highest order. A mean ease with which new channels are created. Vegetation
bifurcation ratio is usually used because the ratio values density is influential in determining drainage density,
for different successive basins will vary slightly. With rel- since it binds the surface layer preventing overland flow
atively homogeneous lithology, the bifurcation ratio is from concentrating along definite lines and from erod-
normally not more than five or less than three. However, ing small rills, which may develop into stream channels.
a value of ten or more is possible in very elongated basins Vegetation slows the rate of overland flow and effectively
where there are narrow, alternating outcrops of soft and stores some of the water for short time periods. Drainage
resistant strata. density also relates to the length of overland flow,