30
A second group of geomorphic conceptual models of river corridors emphasizes
patterns through space. Downstream hydraulic geometry, for example, posits that
discharge is the dominant control on channel geometry and, therefore, channel
parameters such as width, depth, and velocity change progressively downstream as
discharge increases (Leopold and Maddock 1953). Geomorphic process domains
(Montgomery 1999) and river styles (Brierley and Fryirs 2005) also emphasize spatial patterns in river networks.
As emphasis in river management has shifted to restore more natural river corridors, physical models have emphasized the importance of the natural flow and
sediment regimes as primary drivers of river process and form. The natural or altered
water and sediment inputs to a river corridor interact with the geometry and substrate resistance of the corridor to determine the types and stability of channel forms
present (Fig. 2.4).
The natural flow regime refers to the characteristics of the hydrograph present
prior to intensive human alteration of a watershed (Poff et al. 1997) and is commonly described in terms of magnitude, frequency, duration, timing, and rate of
change of flow. Numerous methods exist for quantifying how much an altered
watershed deviates from the natural flow regime (Richter et al. 1996; Poff et al.
2010). Most of these methods compare discharge characteristics of altered rivers to
those of natural rivers. Allowing water to flow downstream in quantities and at times
of the year that mimic a natural flow regime can be difficult and expensive, but is
increasingly being used in river management (Poff and Matthews 2013).
The natural sediment regime refers to the characteristics of sediment inputs,
transport, and storage present prior to intensive human alteration of a watershed
(Wohl et al. 2015). Metrics for quantifying deviation from the natural sediment
regime do not yet exist, partly because of the extremely limited direct measurements of sediment transport and partly because of the greater difficulty in characterizing sediment inputs and transport relative to river discharge. Wohl et al. (2015)
suggest focusing on a balanced sediment regime that involves managing for a
desired balance between sediment supply and transport capacity in order to maintain specific physical processes and forms within the river corridor. Wilcock et al.
(1996) develop a framework for evaluating sediment supply and transport capacity
in relation to maintaining desired channel characteristics. Schmidt and Wilcock
(2008) propose numerical metrics for assessing the presence and magnitude of
sediment surplus or deficit.
In summary, conceptual models of physical process and form in river ecosystems provide a framework within which to characterize the physical integrity of
rivers. This can be useful in understanding how a particular river network or river
segment functions through time and in designing management to restore and
maintain river health.
2 Rivers as Ecosystems
A second group of geomorphic conceptual models of river corridors emphasizes
patterns through space. Downstream hydraulic geometry, for example, posits that
discharge is the dominant control on channel geometry and, therefore, channel
parameters such as width, depth, and velocity change progressively downstream as
discharge increases (Leopold and Maddock 1953). Geomorphic process domains
(Montgomery 1999) and river styles (Brierley and Fryirs 2005) also emphasize spatial patterns in river networks.
As emphasis in river management has shifted to restore more natural river corridors, physical models have emphasized the importance of the natural flow and
sediment regimes as primary drivers of river process and form. The natural or altered
water and sediment inputs to a river corridor interact with the geometry and substrate resistance of the corridor to determine the types and stability of channel forms
present (Fig. 2.4).
The natural flow regime refers to the characteristics of the hydrograph present
prior to intensive human alteration of a watershed (Poff et al. 1997) and is commonly described in terms of magnitude, frequency, duration, timing, and rate of
change of flow. Numerous methods exist for quantifying how much an altered
watershed deviates from the natural flow regime (Richter et al. 1996; Poff et al.
2010). Most of these methods compare discharge characteristics of altered rivers to
those of natural rivers. Allowing water to flow downstream in quantities and at times
of the year that mimic a natural flow regime can be difficult and expensive, but is
increasingly being used in river management (Poff and Matthews 2013).
The natural sediment regime refers to the characteristics of sediment inputs,
transport, and storage present prior to intensive human alteration of a watershed
(Wohl et al. 2015). Metrics for quantifying deviation from the natural sediment
regime do not yet exist, partly because of the extremely limited direct measurements of sediment transport and partly because of the greater difficulty in characterizing sediment inputs and transport relative to river discharge. Wohl et al. (2015)
suggest focusing on a balanced sediment regime that involves managing for a
desired balance between sediment supply and transport capacity in order to maintain specific physical processes and forms within the river corridor. Wilcock et al.
(1996) develop a framework for evaluating sediment supply and transport capacity
in relation to maintaining desired channel characteristics. Schmidt and Wilcock
(2008) propose numerical metrics for assessing the presence and magnitude of
sediment surplus or deficit.
In summary, conceptual models of physical process and form in river ecosystems provide a framework within which to characterize the physical integrity of
rivers. This can be useful in understanding how a particular river network or river
segment functions through time and in designing management to restore and
maintain river health.
2 Rivers as Ecosystems