Geoscience Reference
In-Depth Information
Fig. 11.1 The order numbers of river channel segments in a
natural basin drainage network according to the
Horton-Strahler method.
1
1
1
1
1
1
2
1
2
1
2
1
2
2
3
1
1
1
1
2
3
1
1
2
1
2
3
2
1
2
2
3
3
1
3
first-order streams are considered third-order streams, and so on. The definition of first-
and second-order streams is clear and unambiguous in Horton's procedure, but the def-
inition of third- and higher-order streams required some subjective decisions. To avoid
these and to ensure that only one stream would bear the highest-order number in the
basin, Strahler (1952) adjusted the procedure by stipulating that third-order streams
can only be formed by the joining of any two second-order segments, and so on. The
Horton-Strahler method, as it is now called, is illustrated inFigure 11.1; inthis exam-
ple, there are 18 first-order streams, five second-order streams, and one third-order
stream.
Larger-order river channels usually do not receive much water locally from the riparian
surfaces along their banks, but they receive most of their water from upstream through
lower-order streams. The catchments that are drained by lower-order streams with no
or very few tributaries can be called
headwater
basins,
source area
watersheds, or also
upland
watersheds. Because they feed into channels of progressively higher order, these
lower-order catchments are crucial for a better understanding of runoff mechanisms
in larger basins as well. An important feature for the analysis of runoff from such
headwater catchments is that lower-order river channels tend to have relatively short
residence times; thus any storm runoff hydrograph from a source area watershed is
affected primarily by the nature of the soil mantle areas surrounding the stream and very
little by the nature of the stream itself. Further downstream, however, as more and more
tributaries join, the shape of the hydrograph evolves, and itwill increasingly reflect the
hydraulic characteristics of the channel network. The flow mechanisms inriparian areas
and headwater basins, a topic often referred to also as
hillslope hydrology
, have been the
subject of intense research in the past few decades. A knowledge of these mechanisms
and of their interactions is not only essential to describe streamflow generation, but it is
also the key to a better understanding of solute transport in the human environment and
of the evolution of landforms and erosion.
