An index to assess the extent and success of river and floodplain restoration: Recognising dynamic response trajectories and applying a process‐based approach to managing river recovery

Abstract

How should the success of river and floodplain restoration be assessed? What should we be restoring? We contend that the benchmark for restoration should be the river in its “natural,” that is, quasi‐equilibrium condition prior to deliberate modification. The pre‐modification condition of the river represents a quasi‐equilibrium state in which the river accommodates and adjusts to catchment water and sediment fluxes, whether in a pristine or modified catchment. The resulting assemblage of river landforms (e.g., bars, channels, backwaters) is in balance with the prevailing flood and sediment regime. Furthermore, equilibrium channel forms can adjust to changes in, for example, flood magnitude and frequency via for example, channel expansion or contraction. A restored river system is one that once was anthropogenically restricted, but now has regained capacity to adjust its form. River restoration needs understanding of the history and trajectory of pre‐modification channels, so that the “right” river type can be restored. To assess the scope and success of restoration, we propose using a natural character index (NCI). The NCI is the ratio between a parameter which describes the form and function of a river now (observed) and the same parameter measured at a point or points in time past (expected). Exemplar parameters include sinuosity, floodplain width, active channel width, bar area (at a given flow) and riparian vegetation. Measurement of these parameters utilizes a combination of aerial photos, archive maps and LiDAR‐derived terrain models, the availability of which necessarily limits and determines what parameters can be used in NCI assessment. LiDAR is a useful tool providing an opportunity to comprehend pre‐modification river and floodplain character. We illustrate the NCI concept and its potential for use in assessing restoration effects using examples from New Zealand, Spain and Croatia. The result is an index of use as a “first cut” preliminary assessment for river practitioners.

Proactive river corridor definition: Recommendations for a process‐based width optimization approach illustrated in the context of the coastal Pacific Northwest

“…In scenarios such as this, it may be possible to determine expected meander amplitudes from empirical approaches (e.g., Ackers, 1970; Annable, 1996; Carlston, 1965; Inglis, 1949; Jefferson, 1902; Leopold & Wolman, 1960; Lorenz, 1983; Ward et al, 2002; Williams, 1986) or to use river conditions from before extensive anthropogenic modification or in an analog reach to define an appropriate meander belt width. One such tool to quantify relation to pre‐anthropogenic modifications is the Natural Character Index (NCI) of Fuller et al (2021), who define NCI as the ratio between the value of a parameter which describes the form and function of a river (e.g., sinuosity or corridor width) compared with the pre‐disturbance baseline condition.…”

Riverine landscapes: Challenges and future trends in research and management

“…The need to incorporate historical geomorphic information into river management is emphasized by Fuller et al (2021). They introduce an index for practitioners to assess the scope and success of restoration based on the “natural” condition of a river before anthropic interventions by comparing the river's current and past forms and functions.…”

Measuring geomorphology in river assessment procedures 1: A global overview of current practices

“…Specifically, the streamflow, morphology, and sediment regime are all key factors influencing the local biodiversity (Newson & Large, 2006). Therefore, given the natural spatial and temporal variability in aquatic populations, it may be appropriate to assess changes in physical habitat alongside monitoring population dynamics (Fuller et al, 2021; Kondolf et al, 2007; Rubin et al, 2017). It has, therefore, been argued that river management should incorporate geomorphological concepts because they can contribute to more successful outcomes in terms of river function and habitat quality (Blue, 2018; Biron et al, 2018; Fuller et al, 2019; García et al, 2021).…”