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second challenge is that of understanding the limitations in applying the response
curves. The third challenge is identifying all of the relevant variables at a site.
Quantifying the relationship between control and response variables can be difficult because the relationship is likely to be nonlinear and to include thresholds.
Fish spawning in floodplain areas along the Ob River in Siberia, for example,
requires at least 20 continuous days of overbank flows above a minimum discharge
(Wohl 2011). Flows of smaller magnitude or shorter duration do not allow successful spawning. As another example, significant buffering of sediment and nitrate
fluxes to a channel by riparian zones requires a minimum proportion of total channel length, minimum buffer width, and specific soil composition and plant community composition (Baker et al. 2006; Weller and Baker 2014). Riparian zones that do
not meet these minima may still buffer sediment and nitrate fluxes to the channel,
but the effect will be insignificant. Because thresholds in system function appear to
be ubiquitous in many aspects of river process and form, identification of these
thresholds using response curves can greatly improve the effectiveness of river
restoration.
Limitations in applying response curves can arise from limited data used to
develop the response curve, but a more important source of limitations is likely to
result from the fact that response curves are bivariate, whereas river ecosystems are
multivariate. For example, flow magnitude above some threshold might enhance
fish spawning along the Ob River, but introduced species that feed on fish eggs and
fry might nonetheless limit the successful reproduction of native fish. Consequently,
an important limitation in using response curves is that the control variable being
evaluated may not exert the most important limiting influence on the response
variable.
Most response curves used thus far have focused on flow regime as the master
variable of river ecosystems (Poff et al. 1997) and as the variable that is highly
altered by human activities such as flow regulation. Restoring natural flow regime
may not necessarily restore ecosystem function, however, if other important variables such as sediment regime (Wohl et al. 2015a), invasive organisms, or channel
engineering limit river ecosystem response (Pasternack 2013). Identification of all
relevant control variables can be difficult, but is critical to predicting and managing
river ecosystem response to restoration.
Despite these challenges, response curves form a very useful tool for understanding river ecosystem behavior and are increasingly incorporated in planning environmental flows and other forms of river restoration.
4.5 Metrics of River Health
The European Union adopted a Water Framework Directive in 2000. Among other
things, the directive promoted coordinated river basin management rather than individual countries acting independently to manage their particular portion of a river
basin that crossed national boundaries. The Water Framework Directive also
4 Toward Sustainable Rivers and Water Resources
second challenge is that of understanding the limitations in applying the response
curves. The third challenge is identifying all of the relevant variables at a site.
Quantifying the relationship between control and response variables can be difficult because the relationship is likely to be nonlinear and to include thresholds.
Fish spawning in floodplain areas along the Ob River in Siberia, for example,
requires at least 20 continuous days of overbank flows above a minimum discharge
(Wohl 2011). Flows of smaller magnitude or shorter duration do not allow successful spawning. As another example, significant buffering of sediment and nitrate
fluxes to a channel by riparian zones requires a minimum proportion of total channel length, minimum buffer width, and specific soil composition and plant community composition (Baker et al. 2006; Weller and Baker 2014). Riparian zones that do
not meet these minima may still buffer sediment and nitrate fluxes to the channel,
but the effect will be insignificant. Because thresholds in system function appear to
be ubiquitous in many aspects of river process and form, identification of these
thresholds using response curves can greatly improve the effectiveness of river
restoration.
Limitations in applying response curves can arise from limited data used to
develop the response curve, but a more important source of limitations is likely to
result from the fact that response curves are bivariate, whereas river ecosystems are
multivariate. For example, flow magnitude above some threshold might enhance
fish spawning along the Ob River, but introduced species that feed on fish eggs and
fry might nonetheless limit the successful reproduction of native fish. Consequently,
an important limitation in using response curves is that the control variable being
evaluated may not exert the most important limiting influence on the response
variable.
Most response curves used thus far have focused on flow regime as the master
variable of river ecosystems (Poff et al. 1997) and as the variable that is highly
altered by human activities such as flow regulation. Restoring natural flow regime
may not necessarily restore ecosystem function, however, if other important variables such as sediment regime (Wohl et al. 2015a), invasive organisms, or channel
engineering limit river ecosystem response (Pasternack 2013). Identification of all
relevant control variables can be difficult, but is critical to predicting and managing
river ecosystem response to restoration.
Despite these challenges, response curves form a very useful tool for understanding river ecosystem behavior and are increasingly incorporated in planning environmental flows and other forms of river restoration.
4.5 Metrics of River Health
The European Union adopted a Water Framework Directive in 2000. Among other
things, the directive promoted coordinated river basin management rather than individual countries acting independently to manage their particular portion of a river
basin that crossed national boundaries. The Water Framework Directive also
4 Toward Sustainable Rivers and Water Resources