112
preserving a minimum flow within the active channel in order to sustain fish populations. Early approaches used a biological model of fish habitat preference by
species. The biological model quantified optimal flow depth, velocity, and substrate type. The biological model was combined with a hydraulic model of changes
in habitat availability based on discharge (Bovee and Milhous 1978). Although
this approach does not account for other limitations such as biological competition
or predation or limited physical connectivity, this type of model remains widely
used in evaluating alternative water management scenarios (Stalnaker et al. 1995;
Grigg 2016).
Limitations associated with focusing only on minimum flows led to the idea of
protecting channel maintenance flows. Channel maintenance flows are components
of a river flow regime necessary to maintain specific channel characteristics, such as
cross-sectional area for flood conveyance or substrate grain-size distribution for fish
spawning. The concept of channel maintenance flows explicitly recognizes that discharge variability is necessary to sustain physical and biotic components of a river
ecosystem and that this variability can be characterized in terms of thresholds of
flow magnitude and duration necessary to maintain river processes and forms.
Channel maintenance flows can focus on a limited objective, such as pool scour, or
incorporate a broader range of flow magnitudes designed to maintain a physically
diverse river corridor (Andrews and Nankervis 1995).
Channel maintenance flows designed to maintain multiple aspects of river form
gave rise to the current focus on environmental flows. Environmental flows were
initially experimental flow releases from dams. These flows were designed to restore
specific aspects of the downstream river ecosystem by mimicking naturally occurring floods, although the experimental releases are typically of lower magnitude and
shorter duration than natural floods (Galat et al. 1998; Mürle et al. 2003; Konrad
et al. 2011; Melis 2011; Flessa et al. 2013). Description of the natural flow regime
of a river (Poff et al. 1997) broadened environmental flows from experimental flood
releases to quantifying an annual hydrograph that specifies magnitude, frequency,
timing, duration, and rate of change in flow. Assessing environmental flow requirements typically involves quantifying natural and altered stream flows and changes
in the flow regime (Richter et al. 1996; Gao et al. 2009), as well as quantifying
relationships between hydrologic metrics and physical and biotic river attributes
(Sanderson et al. 2012). Although both of these steps can be challenging, environmental flow assessments are now widely used in a variety of contexts (e.g., Tharme
2003; Arthington et al. 2006; Rathburn et al. 2009; Poff and Zimmerman 2010;
Shafroth et al. 2010; Kozak et al. 2016).
The underlying intent of environmental flows is to reach a compromise between
consumptive water use that takes the entire river flow versus a completely natural
flow regime. This compromise recognizes the need to maintain physical and biotic
characteristics of a river ecosystem while maintaining water supplies for consumptive use (Fig. 4.4).
The need to protect endangered species drives many environmental flows.
Ecologists tend to focus on flow regime, partly because of the enormous influence
4 Toward Sustainable Rivers and Water Resources
preserving a minimum flow within the active channel in order to sustain fish populations. Early approaches used a biological model of fish habitat preference by
species. The biological model quantified optimal flow depth, velocity, and substrate type. The biological model was combined with a hydraulic model of changes
in habitat availability based on discharge (Bovee and Milhous 1978). Although
this approach does not account for other limitations such as biological competition
or predation or limited physical connectivity, this type of model remains widely
used in evaluating alternative water management scenarios (Stalnaker et al. 1995;
Grigg 2016).
Limitations associated with focusing only on minimum flows led to the idea of
protecting channel maintenance flows. Channel maintenance flows are components
of a river flow regime necessary to maintain specific channel characteristics, such as
cross-sectional area for flood conveyance or substrate grain-size distribution for fish
spawning. The concept of channel maintenance flows explicitly recognizes that discharge variability is necessary to sustain physical and biotic components of a river
ecosystem and that this variability can be characterized in terms of thresholds of
flow magnitude and duration necessary to maintain river processes and forms.
Channel maintenance flows can focus on a limited objective, such as pool scour, or
incorporate a broader range of flow magnitudes designed to maintain a physically
diverse river corridor (Andrews and Nankervis 1995).
Channel maintenance flows designed to maintain multiple aspects of river form
gave rise to the current focus on environmental flows. Environmental flows were
initially experimental flow releases from dams. These flows were designed to restore
specific aspects of the downstream river ecosystem by mimicking naturally occurring floods, although the experimental releases are typically of lower magnitude and
shorter duration than natural floods (Galat et al. 1998; Mürle et al. 2003; Konrad
et al. 2011; Melis 2011; Flessa et al. 2013). Description of the natural flow regime
of a river (Poff et al. 1997) broadened environmental flows from experimental flood
releases to quantifying an annual hydrograph that specifies magnitude, frequency,
timing, duration, and rate of change in flow. Assessing environmental flow requirements typically involves quantifying natural and altered stream flows and changes
in the flow regime (Richter et al. 1996; Gao et al. 2009), as well as quantifying
relationships between hydrologic metrics and physical and biotic river attributes
(Sanderson et al. 2012). Although both of these steps can be challenging, environmental flow assessments are now widely used in a variety of contexts (e.g., Tharme
2003; Arthington et al. 2006; Rathburn et al. 2009; Poff and Zimmerman 2010;
Shafroth et al. 2010; Kozak et al. 2016).
The underlying intent of environmental flows is to reach a compromise between
consumptive water use that takes the entire river flow versus a completely natural
flow regime. This compromise recognizes the need to maintain physical and biotic
characteristics of a river ecosystem while maintaining water supplies for consumptive use (Fig. 4.4).
The need to protect endangered species drives many environmental flows.
Ecologists tend to focus on flow regime, partly because of the enormous influence
4 Toward Sustainable Rivers and Water Resources