Mountain Rivers and Incised Channels - River Dynamics and Integrated River Management - page 141

3.3 Bed Structures Resisting Incision

3.3.1 Step-pool Systems
A step-pool system is a geomorphologic phenomenon occurring in high-gradient (>3 5%) mountain
streams with alternating steps and pools having a stair-like appearance (Chin, 1999). The step-pool
system usually occurs on a stream with bed materials consisting of particles with diameters differing by
several orders of magnitude with the largest diameter on the same order as the water depth. Cobbles and
boulders generally compose the steps, which alternate with finer sediments in pools to produce a
repetitive, staircase like longitudinal profile in the stream channel, as shown in Fig. 3.21.

Fig. 3.21 Staircase-like longitudinal profile of a step-pool system in high gradient stream channels

A step-pool system develops usually in small mountain streams with a several meters wide channel.
Figure 3.22(a) shows the step-pool system in a small ravine in the upper Yangtze River basin, which has
a channel width of only 2 m. Large stones play an important role in the development of a step-pool
system. In relative large streams with high gradients, step-pool systems may develop if there are huge
stones. Figure 3.22(b) shows the step-pool system on the Xiaojinchuan River in Sichuan Province, which
has a channel width of several tens of meters. The large stones have a diameter of more than 5 m. The
structure developed after a landslide and long term erosion. The step-pool system causes a water head
reduction of more than 50 m and composes a knickpoint of the Xiaojinchuan River.
The tight interlocking of particles in steps gives them an inherent stability that only extreme floods are
likely to disturb, which suggests that step-pools are a valid equilibrium form, especially when coupled
with their apparent regularity and their role in satisfying the extreme condition of resistance maximization.
Step-pool systems also develop in many mountain streams in China, such as the Yuzixi and Zagunao
rivers, tributaries of the Minjiang River and the Jiuzhai Creek in Sichuan Province, the Qingshui River in
Guizhou Province, and the Shengou Ravine and tributaries of the Xiaojiang River in Yunnan Province.
There are many research results on the hydraulic features of regular step-pool systems. The step-pool
system affects not only flow resistance but also sediment transport, which in flume experiments occurs as
a series of waves linked to the underlying bed morphology (Whittaker, 1987, Rosport, 1994). Their role
as energy dissipaters can be impaired when pools become filled with sediment (Whittaker and Jaeggi
Martin, 1982), for then there is an increase in velocity and erosive capability, reactions that are opposite
to the original formation of mechanism the step-pool system. The bed adjustment in a step-pool system
in Leinbach, Germany shows the function of increasing flow resistance (Ergenzinger, 1992). The large
boulders in the steps act as a framework tightly interlocking the structure resulting in considerable
stability. Given the need for one or more keystones, the development of a step is strongly influenced by
local sediment supply and transport conditions. The pools between steps provide storage sites for finer
bed material.
Step-pool morphology can be characterized by two variables: step wavelength L and step height H s , so
that H s / L is an index of step steepness and bears a close relation to the loss of head per unit length of