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 Fundamentals of Fluvial Geomorphology and Channel Processes
Figure 2.14 Lane's Balance (after E. W. Lane, from W. Borland)
obtaining additional length elsewhere through meandering, with the net result being that the
overall reach length, and therefore slope, remains unchanged.
In summary, a stable river, from a geomorphic perspective, is one that has adjusted
its width, depth, and slope such that there is no significant aggradation or degradation of the
stream bed or significant plan form changes (meandering to braided, etc.) within the
engineering time frame (generally less than about 50 years). By this definition, a stable river
is not in a static condition, but rather is in a state of dynamic equilibrium where it is free to
adjust laterally through bank erosion and bar building. This geomorphic definition of stability
(dynamic equilibrium) is developed here to establish a reference point for the discussion of
system and local instability in the following sections.
2.2.2 SYSTEM INSTABILITY
The equilibrium of a river system can be disrupted by various factors. Once this occurs
the channel will attempt to re-gain equilibrium by making adjustments in the dependent varia-
bles. These adjustments are generally reflected in channel aggradation (increasing bed
elevation), degradation (decreasing bed elevation), or changes in planform characteristics
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