Geology Reference
In-Depth Information
( ) Stream order (Strahler)
( ) Stream magnitude (Shreve)
a
b
1
1
1
1
2
4
18
3
2
2
1
1
15
14
4
2
2
13
1
1
1
1
1
1
11
4
1
1
1
1
2
2
5
3
1
1
1
6
1
3
1
3
1
2
2
2
4
3
2
2
2
2
1
1
2
2
1
1
1
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
distance and all fourth-order basins are considered as
similar.
Shreve's ordering system
, on the other hand,
defines the magnitude of a channel segment as the
total number of tributaries that feed it. Stream mag-
nitude is closely related to the proportion of the total
basin area contributing runoff, and so it provides a
good estimate of relative stream discharge for small river
systems.
Strahler's stream order has been applied to many river
systems and it has been proved statistically to be related to
a number of drainage-basin morphometry elements. For
instance, the mean stream gradients of each order approx-
imate an inverse geometric series, in which the first term
is the mean gradient of first-order streams. A commonly
used topological property is the
bifurcation ratio
, that is,
the ratio between the number of stream segments of one
order and the number of the next-highest order. A mean
bifurcation ratio is usually used because the ratio values
for different successive basins will vary slightly. With rel-
atively homogeneous lithology, the bifurcation ratio is
normally not more than five or less than three. However,
a value of ten or more is possible in very elongated basins
where there are narrow, alternating outcrops of soft and
resistant strata.
The main geometrical properties of stream networks
and drainage basins are listed in Table 9.1. The most
important of these is probably drainage density, which is
the average length of channel per unit area of drainage
basin.
Drainage density
is a measure of how frequently
streams occur on the land surface. It reflects a balance
between erosive forces and the resistance of the ground
surface, and is therefore related closely to climate, lithol-
ogy, and vegetation. Drainage densities can range from
less than 5 km/km
2
when slopes are gentle, rainfall low,
and bedrock permeable (e.g. sandstones), to much larger
values of more than 500 km/km
2
in upland areas where
rocks are impermeable, slopes are steep, and rainfall totals
are high (e.g. on unvegetated clay 'badlands' - Plate 9.5).
Climate is important in basins of very high drainage den-
sities in some semi-arid environments that seem to result
from the prevalence of surface runoff and the relative
ease with which new channels are created. Vegetation
density is influential in determining drainage density,
since it binds the surface layer preventing overland flow
from concentrating along definite lines and from erod-
ing small rills, which may develop into stream channels.
Vegetation slows the rate of overland flow and effectively
stores some of the water for short time periods. Drainage
density also relates to the length of overland flow,
