Basic Concepts and Management Issues of Rivers - River Dynamics and Integrated River Management - page 3

opened the study and understanding of this complex geomorphic system. Horton's approach to network
description was applied and expanded by the "Columbia School" of A. N. Strahler and his students, and
many studies of drainage basins and stream networks have followed the lead of Horton and Strahler.
Stream ordering —Horton (1945) developed a method of classifying, or ordering, the hierarchy of
natural channels within a watershed, and the modified system of Strahler (1957) is probably the most
popular today. Horton-Strahler's stream ordering system is portrayed in Fig. 1.3(a). The uppermost
channels in a drainage network (i.e., headwater channels with no upstream tributaries) are designated as
the first-order streams down to their first confluence. A second-order stream is formed below the
confluence of two first-order channels. Third-order streams are created when two second-order channels
join, and so on. In the figure the intersection of a channel with another channel of lower order does not raise
the order of the stream below the intersection (e.g., a fourth-order stream intersecting with a second-order
stream is still a fourth-order stream below the intersection). Within a given drainage basin, stream order
correlates well with other basin parameters, such as drainage area or channel length. Consequently, knowing
what order a stream is can provide clues concerning other characteristics such as which longitudinal zone
it resides in and relative channel size and depth.
The stream ordering method by Horton and Straheler is very useful for theoretical analysis but not
practically useful for engineering studies because one cannot identify the first order streams in a watershed.
In engineering practice the channels are ordered in the following method as shown in Fig. 1.3(b): the
channels flowing into the stem river, which flows into the ocean, are the first order tributary of the river;
the channels flowing into one of the first order tributaries are the second order tributary; and the channels
flowing into one of the second order tributaries are the third order tributary; etc..

Fig. 1.3 Stream orderings in a drainage network (a) Horton-Strahler's stream ordering system; (b) Engineering stream
ordering system

Horton's Laws Horton (1945) pioneered quantitative studies of river morphology by introducing his
laws of stream number, N m , stream length, L m , stream area, A m , and stream slope, s m ; i.e.,
e B NA Z

Z

(1.1)

LC Z e D

Z

(1.2)

s E Z e

F

(1.3)

Z

AG Z e H

Z

(1.4)