1. Introduction

Human societies since the dawn of civilization have grown near, altered, and utilized river resources heavily for a variety of socio-economic functions (Palmer & Ruhi, 2019). Until recently, rivers have been managed similarly to other natural resources where tensions between stakeholders are mitigated with legal structures, and the resource is aimed to be used in a manner that provides maximum societal benefit (Lane-Miller et al., 2013). Society’s relationship with river resources has historically, and nearly ubiquitously, caused the degradation of riverine ecosystems, which here are considered to be the silent stakeholders affected by river management decisions (Down, 2019). Humans have harmed riverine ecosystems by fundamentally changing the habitat said ecosystems evolved to be adapted for along two major, but interconnected, axes; altering flows, and altering physical habitat (Giller, 2005). More recently, societies have begun to increasingly value ecological systems (Wohl et al., 2005) and recognize the scale of the environmental harm inflicted in relative ignorance (Down, 2019). Where human population densities are highest, domestic and invasive species better suited for the banal channelized rivers flowing without many critical aspects of their natural flow patterns (Figure 1) far outnumber native biota (Johnson et al., 2019). The mechanisms causing this to be the case, along with the approaches applied to mitigate ecological degradation are reviewed in this paper within the context of how science can guide future ecological restoration.

Figure 1. Typical effects of different levels of human population density on streams (Johnson et al., 2019).

2. Environmental flows

The term environmental flows originally referred to a quantity of water left in a river channel, within the context of deciding minimum flow regulations, but today generally includes notions of both water quantity, timing, and sometimes quality (Acreman, 2016). Lack of flow quantity due to low reservoir releases or excessive upstream water uses dramatically reduces the volume of aquatic habitat, concentrates pollutants, alters biotic selective pressures, and generally lacks the velocity necessary to transport fine-grain sediments downstream (Acreman & Dunbar, 2004).

2.1 The natural flow regime

Concepts of flow regime capture the annual and inter-annual hydrologic variability of a river and is intricately related to both channel-morphology and ecosystem processes (Poff et al., 1997). Metrics such as annual average discharge fail to capture a river’s flow intra- and inter-annual discharge variability which functions as a probability distribution for each day of the year and can change with shifts in climate or by human alterations (Poff et al., 1997., Lane et al., 2018). The natural flow regime of a river, assuming no human alteration, is an ecologic forcing as the presence or absence of high flows, as well as their intra/inter-annual timing exerts selective pressures on organisms causing most riverine species to be specifically adapted to the flow regimes in which they are native (Poff et al., 2010., Palmer & Ruhi, 2019). Therefore, altering a flow regime whether intentionally or not almost always negatively affects the species that have adapted to it (Poff & Zimmerman, 2010., Christer & Svedmark, 2002). An explicit example of ecologic flow regime reliance is the multitudes of fish species that use high flows to cue seasonal change, and therefore trigger migration (Palmer & Ruhi, 2019). Spatially variable floodplain-channel topography interacts with the temporally variable flows to provide critical ‘services’ that many riverine species rely on (Yarnell et al., 2015). Flow regime determines the intensity and frequency of floodplain accessing flows, which in turn control the timing of nutrient cycling in and out of the channel as well as the ‘flushing’ of fine-grained sediment down system which allows fish species to lay their eggs among the previously buried, more stable, coarse sediments (Palmer & Ruhi, 2019). The frequency of floodplain-accessing high flows also determines the success of riparian plant communities, which in turn facilitate the cycling of organic matter into the channel, which is consumed by a variety of biota (Christer & Svedmark, 2002).

Direct river developments and proximal economic land uses can alter a rivers flow regime substantially (Poff & Zimmerman, 2010). The construction of dams can all but erase seasonal flow peaks, and water diversion dramatically lower the annual river discharge (Bernhardt & Palmer, 2011). Impermeable land uses within a river’s watershed can increase the flashiness of peak flows, causing unsustainable scour of the coarse sediments species rely on and reducing the duration of high flows which alters selective pressures (Down, 2019). The ecological relevance of natural flow regimes is underscored by the ubiquity of ecological damage in response to altered flow regimes (Figure 1). A review of 165 papers assessing flow alteration’s effects on ecology found that 92% of the sampled rivers recorded substantial decreases in ecological metrics (Poff & Zimmerman, 2010). Anthropogenic flow alterations affect native ecology not only through limiting critical processes but also simply by changing sub-lethal flow patterns which function as decisive selective pressure controlling relative success of numerous riverine species (Poff & Zimmerman, 2010). This is reflected in many highly altered reaches where total biomass may decrease slightly, while relative species abundance shifts decisively to invasive or domestic species which are better suited for the altered regime (Johnson et al., 2019). Mitigating the damage caused to the environment by both direct and indirect river alterations boils down to providing adequate minimum discharges, and where possible, attempting to restore the ecologically relevant aspects of a given flow regime.

2.2 Implementing environmental flows

Water is secured for environmental flows via either restrictive or active management (Acreman & Dunbar, 2004). Restrictive management refers to cutting down on upstream diversions, often through water buyback or mandate, to maintain a determined discharge and almost always involves friction with the beneficiaries of economic water uses (see section 4.3). In the majority of cases, restrictive management is limited to providing a determined minimum environmental flow, the magnitude of which becomes instantly debated due to its socio-economic implications. Science-led approaches to determining environmental flow magnitude attempt to optimize ecological benefit at the lowest required flow possible (Figure 3). One of the most commonly applied environmental flow prescribing approaches is the instream flow incremental methodology (IFIM) which involves using physical habitat simulation (PHABSIM) to model one-dimensional habitat area as a function of flow to find possible optimals (Hartfield & Bruce, 2000). This approach, as well as others (see section 4.1), rest on the assumption that habitat quality as defined by ranges of substrate and hydraulics is a suitable proxy for fish and biomass viability. The IFIM can optimize habitat to a variety of discharge-habitat curves used to represent different species and ecosystems, yet is limited to providing a suggested optimal flow amount, paying little attention to aspects of flow regime which affects ecological selective pressures, migration cueing, and other relevant processes (Poff et al., 2010). The IFIM’s reliance on one-dimensional habitat modeling, as well as the many other applied environmental flow methodologies (EMFs) that use PHABSIM, causes two-dimensional habitat variability, which can be strongly incongruent with any one-dimensional representation, to be systematically overlooked (Moir & Pasternack, 2008).

Figure 3. A simple habitat-flow curve, with optimal being most habitat area at the lowest flow (Hartfield & Bruce, 2000).

Active management techniques are implemented in reservoir controlled rivers, where reservoir operation rules can be manipulated to provide suitable environmental flows (Acreman, 2016). In contrast to restrictive management, active management generally involves lower economic costs and allows for environmental flows to be temporally variable. As mentioned previously, the flow patterns and floodplain-accessing flows both serve critical ecological services, which can be replicated to some degree utilizing reservoir operations (Acreman & Dunbar, 2004). How to most effectively gain the ecological benefits known to be present within a natural flow regime while not being able to return to the full pre-development discharges is currently a researched topic. One well-supported approach is releasing a percent of a determined ‘natural flow regime’(Poff et al., 1997), which rests on the assumptions that the ecological benefit of environmental flows increases linearly with similarity to the natural regime and that amidst a highly altered environment, scientists can discern what the natural hydrologic baseline is (Lane et al., 2018). A large amount of approaches to estimate hydrologic baseline exists (Lane et al., 2018), but in many analyses, the natural flow regime is estimated by looking at the history of local alterations and approximating how the present alterations may be changing an annual hydrograph (Figure 2)(Acreman, 2016). Historical analyses are limited in regions where rivers have been altered for long periods of time and any pre-historical flow regime existed under different climatic forcings such as in Western Europe or the Middle East (Acreman et al., 2014). More recently, headwater gauges and computational classification techniques have been applied to automatically predict natural flow regime conditions in the form of a dimensionless hydrologic archetype that can be used to generalize between rivers, drastically reducing the time necessary to develop a hydrologic baseline (Lane et al., 2019). Although likely to provide significant ecological benefit (Acreman et al., 2014), attempting to replicate a percent of a natural flow regime by manipulating reservoir releases can result in overly complex release rules that lack specific scientific justification, potentially lowering their appeal to reservoir operators (Lane-Miller et al., 2013).

Figure 2. Dimensionless natural flow regime archetypes present in California, notice variation in the intensity, frequency, and annual timing of peak flows.

In conflict with the assumption that anything closer to the natural regime is preferable is the geometry of river channels, which typically exhibit sudden terrace or floodplain forming slope breaks (Pasternack et al., 2018), therefore any attempt to gain the ecological benefits of floodplain accessing flow must consider what specific discharge threshold is required by a channel’s topography. The ‘functional flows’ approach to active management attempts to address flow-topography thresholds while minimizing the complexity of the reservoir operations and annual frequency of flushing flows which are associated with non-negligible missed opportunity costs to hydro-electric dams (Gomez et al., 2014., Yarnell et al., 2015). In essence, functional flows aim to target specific high-discharge releases with the most ecological and geomorphic relevance on annual and inter-annual cycles (Yarnell et al., 2015), which requires less active reservoir release management and in theory can access similar ecological benefits. Yarnell et al (2015) propose three annual ‘function’ high flows with the largest being a wet-season peak, followed by a smaller but longer duration spring recession flow which aims to replicate snowmelt flux, and finally a small wet-season initiation flow in November for Mediterranean climates. The functional flow approach appears promising, although studies will need to define the specific ecological functions of different parts of natural flow regimes to generalize the approach to other climate zones.

3. River Restoration

3.1 Concepts and approaches

The second aspect of the traditional ecological restoration paradigm is the quality, and availability of physical habitat suitable for desired biological activity. Riverine species rely upon habitat that is often smaller in area than terrestrial and marine organisms. In addition, due to society’s reliance on rivers for economic activity, the desire to reduce flooding risk, and watershed scale drainage processes, riverine habitat is strongly affected, both directly and indirectly, by humans. Anthropogenic economic activities, as well as direct river alterations, have dramatically degraded the quality of physical habitat in rivers all over the world (Bernhardt & Palmer, 2011, Downs, 2019, Yoshimura et al., 2005). Channelization of rivers reduces the total area of potential habitat while increasing scour of biologically necessary sediment and limiting morphological processes, such as lateral migration or braiding, which are intertwined with physical habitat creation and maintenance (Down, 2019). Previously discussed flow alterations, such as increased peak flood stages due to impermeable land use, can cause habitat degradation via excessive sediment scour as well (Poff et al., 2010). In addition to their various effects on ecologically relevant flow regime characteristics, the construction of dams traps coarse sediment biasing deposition towards smaller clasts as well as limiting the natural building of bars and other morphological units (Kondolf et al., 2019). Regardless of flow regime, the removal of flood plains and riparian zones decrease the amount of woody matter available to use as habitat, increase flood stage velocities while erasing potential refuge areas, and increase water temperature via decreasing tree cover (Wohl et al., 2005). Societal awareness and valuation of riverine ecosystems and the services they offer society (Giller, 2005) has increased in tangent with the political desire to reverse ecosystem degradation and mitigate future anthropogenic impacts with physical river restoration (Bernhardt & Palmer, 2011). River restoration approaches are diverse and have evolved with time. Due to river restoration existing as the main application, and therefore litmus test for sub-fields of river science such as ecohydraulics, it has been extensively researched in the past decades. An overview of river restoration from the philosophy of ecological goal setting, the contemporary understanding of riverine habitat variables and effects of restoration approaches, as well as future applications of recent research advancements will be discussed in the following sections.

Aesthetically focused river ‘restoration’ applying construction equipment to make a channel appear more natural and appealing for recreational use has occurred with minimal scientific context where political will and capital is available mostly since the 1980s (Wohl, 2005). Societal awareness and valuation of ecosystem services has increased since then (Palmer et al., 2005), and aims of the increasingly well-funded restoration projects have shifted towards environmental stewardship and the revitalization of native ecologies. Yet despite having an empirical notion of what a successful river restoration project should bring about (i.e. increase in native fish population), the projects of the late 20th century generally lacked scientific guidance and any mechanism for gauging success past the pure aesthetics that were already valued (Giller, 2005, Kondolf et al., 2007). Simplistic conceptualizations, that remained untested significantly into the era of their widespread use, of ecological-morphology relationships guided almost all of the earlier restoration efforts which became especially popular in Western Europe (Bernhardt & Palmer, 2011). The most common of such approaches was placing additional structures instream, often rocks or woody material, to increase flow velocity heterogeneity and provides more diverse habitat types as well as high flow refuge areas (Figure 5). Miller & Kochel (2012) studied records of 26 American restoration projects and found that of the 558 evaluated channel changes, 391 were the addition of in-stream features. While this approach does increase flow heterogeneity and therefore ecological suitability in the short term, Miller & Kochel found that in only 7 years most of the instream additions were degraded past the point of usefulness in 10 of the 26 sites. This suggests that the durability of channel reconstruction in the face of channel processes has been hardly considered, which renders many of the world’s past restoration attempts ecologically useless in the long run without long term, expensive, maintenance. Yet this fact is hardly reflected in the planning of restoration projects, of which the majority lack consideration of channel processes and don’t include any mechanism or funding to assess long term success rates (Kondolf et al., 2007, Kondolf et al., 2008, Palmer et al., 2005).

3.2 Complexities and assessment

It is increasingly clear that if river restoration projects are to be funded, their needs to a higher level of sophistication to project design that can both increase economic efficiency and ecologic results. Identifying clear, testable, and achievable project goals lays the groundwork for an empirically based restoration effort and will also provide valuable data to the scientific community that allows further refinements of project approaches. Ecological nuances, network effects, and population bottlenecks are necessary to consider when designing such goals. Most river degradation is either from major landscape alterations of lack of necessary environmental flows (Bernhardt & Palmer, 2011), therefore using resources to improve physical habitat can be a total waste of monetary and political capital since without addressing the strongest negative ecologic forcing a project is doomed to fail and potentially fail publically weakening the social will to continue to fund similar projects. In addition, proper environmental flows, and excellent habitat restoration can also fail to reap an ecologic reward if there is no connectivity to a potential colonist population (Sundermann et al., 2011). In fact, a restored river reaches proximity and connectivity to existing ecological populations is a strong determinant of restoration success, as populations will fail to revitalize an area if their path to the restored habitat is impinged by physical alterations such as dams or stretches of highly degraded habitat. While fish passageways around dams have been engineered to mitigate the former issue to mixed results (Birnie-Gauvin et al., 2018, Pompeu et al., 2011), it remains unrealistic to expect a population recovery at a given restored reach if it is surrounded by highly unsuitable habitat. In addition, the river restoration itself can have negative effects on ecosystems. Projects in which trees are cut to allow heavy machinery to reach the river can reduce shade and lead to less favorable warmer water. The disturbance of the construction can act as a selective pressure temporarily preferring more disturbance resistant taxa (Bernhardt & Palmer, 2011). Numerous experts posit that testable goals must be established before restoration begins, and monitoring should be done before, during, and after the project with annual check-ups to evaluate the degree of success or failure (Jahnig et al., 2011, Morandi et al., 2014, Giller, 2005, Woosley et al., 2007). Semi-continuous monitoring after the project is complete is especially important. Interannual variance species populations and diversity are substantial (Hockendorff et al., 2017), rendering only one post-restoration sampling incapable of capturing the reality of a project’s effects. Sampling post-project is also essential to monitor the degree of self-sustenance any restoration alterations may have, which can further inform the restoration community, although ideally, a project should be resilient to external forcings by working with the channel morphological processes (Palmer et al., 2005). Morandi et al (2014) argue that continuous monitoring of the restored site is not enough, and discontinuous monitoring of unrestored, simular, control site is necessary to empirically judge success relative to how the reach may have behaved without restoration.

How to define restoration success is a debated subject, and is always relative to the physical and social environments the project exists in. Creating sets of quantifiable indicators, from which a team could select from and weight, is one promising framework allowing both flexibility in goal setting but also some degree of standardization (Woolsey et al., 2007). Others affirm that restoration goals can be standardized further via river classification schemes that can ascribe generalize indices of success within a class (Wohl et al., 2005). For example, a stream classified as highly urbanized can be restored with regards to flood risk and aesthetics while a river classified as wild and ecologically productive can have its success measured by indices of species population, water quality, and riparian forest density. In addition, Palmer et al (2005) point out that due to society’s valuation of restoration, which comes not only from ecologic population recovery but also aesthetic, recreation, and economic benefits, multiple definitions of success must be incorporated into the project design to assure political viability. Research showing indirect benefits of river restoration, such as substantial and temporally durable reduction in peak flood stages, may increase restorations desirability as a “soft engineering” approach to flood damage mitigation (Dixon et al., 2016) and raise valuation metrics such as household willingness to pay which are frequently used to decide restoration budgets (Lee, 2012).

4. The future of ecological restoration efforts

4.1 Application of ecohydraulics

One promising field of inquiry, that may give both river restoration environmental flows a valuable scientific framework, is ecohydraulics which mechanistically links fluvial geomorphology and riverine ecology generally with 2D or 3D modeling techniques where habitat within the channel can be idealized as ranges certain hydraulic characteristics. Developments in ecohydraulics are poised to give analytical rigor to river restoration projects where potential channel morphology alterations can be simulated in regards to specific ecological functions, and designed to be in-synch with channel processes, and therefore resistant to the degradation that plagues many past projects (Miller & Kochel, 2012). Ecohydraulics has developed out of a desire to increase the sophistication of aquatic habitat assessment techniques such as PHABSIM, and can provide a more nuanced picture of microhabitat changes at different flow levels or channel morphologies, which enables more resource-efficient river restoration approaches.

In the context of environmental flows, ecohydraulics offers a more rigorous way to optimize habitat availability and quality by modeling and quantifying responses to different flow levels past the more traditionally applied concepts of minimum depth and floodplain connectivity (Christer & Svedmark, 2002., Thomson et al., 2001). Early on, Statzner et al. (1988) observed hydraulic velocity’s profound effects on the metabolism of all riverine species, especially filter feeders, as well as other key ecological processes and demonstrated the need for the study of “hydraulic stream ecology”. The hydraulic environment created at the nexus of flow regime and channel morphology, generally contains spatially variable areas of different hydraulic characteristics, which are defined by mean velocity, water surface slope, depth, bottom roughness, kinematic viscosity, and control the distribution of resources aquatic organisms rely on as well as the bio-energetic costs associated with staying stationary in different locations (Statzner et al., 1988., Thomson et al., 2001). Hydraulics influence sediment transport, diffusion of oxygen, chemical reactions, and water temperature (Thomson et al., 2001). Sediment transport is of particular importance, as it defines substrate composition which is related to the distribution of benthic organisms as well as stages of fish life cycles where eggs must be buried and protected by larger clasts (Thomson et al., 2001). Therefore ecohydraulics is essentially the study of habitat-flow relationships at a granular scale (Rice et al., 2010) via two or three-dimensional modeling of channel hydraulics, which relies on the availability of high-resolution channel elevation data. In the past, the difficulty of acquiring quality channel elevation data limited the applicability of ecohydraulic modeling to real-world river restoration or environmental flow prescription, yet recent advances in green-LiDAR, which can capture sub-meter scale bathymetric topography, appear to dramatically expand the practicality of the field (Kinzel et al., 2012). Different environmental flow regimes can be “tested” via 2D models, with depth and velocity outputs at the resolution of the input elevation data, which in turn can be manipulated with know ecologic and geomorphic equations. In physical river restoration, ecohydraulic approaches offer the ability to directly model proposed channel alterations or the placement of instream features and their effect on habitat availability (Rice et al., 2010). Paired

4.2 Limitations and promise of ecohydraulic approaches

Ecohydraulic approaches to river restoration and environmental flows share the core limitation of past approaches; lack of ground truth. Whether habitat suitability is modeled down a channel longitudinally such as in previously applied environmental flow management techniques, or two-dimensionally using ecohydraulics, or not at all as in many restoration projects, it is still being used as a proxy for population viability, which all applied techniques fail to measure directly (Shenton et al., 2012). Bio-verification techniques, which attempt to assess the degree to which modeled suitable habitat is used by organisms, are promising and when applied have demonstrated adequate correlations between modeled ecological viability and actual organism locations (Kammel et al., 2016). Sediment dynamics, especially in the long term, as well as climate changes potential alteration of ecosystems and flow regime are two areas where lack of scientific understanding is limiting the ability to design physically and climate outcome resilient restoration projects (Wohl et al., 2015).

Ecohydraulic modeling is poised to provide exponentially more value to river restoration and environmental flow efforts as computational processing power and the proportion of river networks mapped with bathymetric LiDAR increases. A bright future is imaginable where network scale 2D hydraulic models can provide depth and velocity values at high resolution across hundreds of miles of rivers allowing accurate and spatially expansive analysis of habitat quality, diversity, and connectivity. Such data would be invaluable for prescribing environmental flows, as dam releases or consumption points could be optimized to balance societal demands and network-wide habitat availability. Issues plaguing river restoration, such as habitat connectivity bottlenecks that limit the effectiveness of many restoration projects (Bernhardt & Palmer, 2011), could be empirically discovered and future projects could be designed and located strategically to optimize a given amount of political or economic capitals ecological contribution to the river network as a whole. Increasing the ecological reliability of river restoration efforts, as well as convincingly quantifying environmental flows effects on a network, could expand the political viability of said efforts and more authoritatively convey the state of our river ecosystems, as well as exactly what we could do about it, to a skeptical public audience.

4.3 Socio-economic considerations

Restoring our river networks and their ecological systems to a pre-industrial state is impossible as many human alterations are necessary for society to function with reliable access to water and protection from floods, but the tools to enter an era of sophisticated environmental stewardship are within reach (Downs, 2019). The variables that will constrain any transition towards river management that effectively mitigates and reverses harms done to silent ecological ‘stakeholders’ are primarily very human. Some level of political and capital to physically restore riverine habitat is present today, but is far from assured, and will need to be expanded in the face of abysmal ecological realities, such as extinctions and large invasive populations, which are related to severe lack of suitable native physical habitat. The securing of water for environmental flows via restrictive management is intrinsically a socio-economic proposition that is generally met with resistance regardless and can be limited by a goverment entity’s inability to acquire the necessary water rights (Poff et al., 2003). Policy advancements allowing for more liquid water markets and ‘trial’ water rights transfers could expand water buyback techniques and allow environmental flows to be secured in non-reservoir controlled reaches (Lane-Miller et al., 2013). Yet buyback schemes may be limited in market scenarios where stakeholder’s missed opportunity costs associated with selling water rights are significantly higher than what a government entity or Non-Government Organization could afford to pay. But regardless of buyback scheme, risk of water shortage maintains present, and restrictive management decisions must address how shortage flows are distributed, prioritizing either environmental, economic, or domestic uses.

Active management for environmental flows is not free of socio-economic restraints either. Altering dam release rules to replicate natural flow regime patterns or acheive functional flows can have negative effects on hydropower generation. In Spain’s Lower Ebro River, cost-benefit analysis of using reservoir operations to provide two floodplain-accessing flushing flows a year estimate a missed opportunity cost at 0.17% of yearly revenue which would be not entirely insignificant at larger facilities, although altering reservoir release appears to have significantly lower economic costs than acquiring water from elsewhere (Gomez et al., 2014). Coupling actively managed environmental flows and floodplain-channel alterations allows for the full ecological benefit of either approach to be realized (Figure 5). For example, a released functional high flow will fail to provide the intended floodplain connectivity if the river is channelized or excessively scoured (Down, 2019., Whipple & Viers, 2019). But in practice the management of river flows and riparian land is often disjointed, acting as a bureaucratic speed bump limiting implementation of the most beneficial types of coupled strategies (Whipple & Viers, 2019).

Figure 5. Graphic illustrating how either environmental flows or river restoration can have ecological benefits, but a coupled approach offers potential for greater benefit (Whipple & Viers, 2019).

Conclusion:

Society values riverine ecosystems and is willing to invest in their restoration, but expanding investment will be met with higher expectations of success (Poff et al., 2003). The scientific literature has remained more sophisticated than most implemented restoration projects and environmental flow policies (Palmer et al., 2005., Acreman et al., 2014), which is in part to blame for mixed ecologic outcomes (Wohl et al., 2005). The growing body of literature outlining flow timing and floodplain connectivity’s intricate relationships with ecologically relevant physical and chemical processes should be enough to predicate a more nuanced societal approach to ecological restoration plans. Contemporary ecohydraulic 2D modeling enables the type of granular analysis required to test and design both flow management techniques and restoration project design that utilize known ecology-flow relationships (Shenton et al., 2011). A future is feasible in which modern societies enter a phase of science-led environmental stewardship that addresses the thousands of rivers worldwide that are far from an ecologically vital state (Down, 2019). What such as future means at the river scale depends largely on the state of alteration, defined ecological goals, and capital available. In some basins, the securing of water rights is the best realistic option for providing suitable minimum environmental flows, while in other reservoir controlled rivers strategic release rules such as functional flows could replicate critical parts of the natural flow regime and evolve with advancements in the understanding of such regimes (Yarnell et al., 2015). Physical habitat can and should continue to be restored, but project designs without plans addressing post-restoration monitoring, channel modification resilience, and the specific habitat needs of the desired native species should cease to be funded. Instead, ecohydraulic modelling paired with digital channel design should quantitatively guide channel modification in order to maximize ecological services and alteration resilience (Hardy et al., 2016., Wohl, et al., 2005). Flow regime manipulation and channel restoration projects should at a minimum be in communication with each other (Poff et al., 2003) but ideally be applied together as a coupled approach, which can increase both project cost-effectiveness and optionality (Whipple & Veirs, 2019). Conceptual linking of flow patterns (Shenton et al., 2012), channel morphologies, and ecological communities can expand the generalizability of restoration approaches while advancements in ecohydraulic modeling as well as project monitoring techniques would add ecological value to the approaches themselves (Wohl et al., 2005).

References:

Acreman, M. 2016. Environmental flows: Basics for novices. WIREs Water, 3, 622–628.

Acreman, M.C., and Dunbar, M.J. 2004. Defining environmental river flow requirements? A review. Hydrology and Earth System Sciences Discussions, 8(5), 861–876.

Acreman, M. C., Overton, I.C., King, J., et al. 2014. The changing role of ecohydrological science in guiding environmental flows. In Special issue: Hydrological science for environmental flows. Hydrological Sciences Journal, 59(3–4), 433–450.

Bernhardt, E.S., and Palmer, M.A. 2011. River restoration: the fuzzy logic of repairing reaches to reverse catchment scale degradation. Ecological Applications, 21(6), 1926–1931.

Birnie‐Gauvin, K., Franklin, P., Wilkes, M., and Aarestrup, K. 2018. Moving beyond fitting fish into equations: Progressing the fish passage debate in the Anthropocene. Aquatic Conservation: Marine and Freshwater Ecosystems, 29(7), 1095–1105.

Christer, N., and Svedmark, M. 2002. Basic principles and ecological consequences of changing water regimes: Riparian plant communities. Environmental Management, 30(4), 468–480.

Dixon, S.J., Sear, D.A., Odoni, N.A., Sykes, T., and Lane, S.N. 2016. The effects of river restoration on catchment scale flood risk and flood hydrology. Earth Surface Processes and Landforms, 41, 997–1008.

Downs, P. (2019, October 29). Parallel trends in river evolution for the Anthropocene: implications for river research and management. Seminar presented at the University of Davis, CA, USA.

Giller, P.S. 2005. River restoration: seeking ecological standards. Journal of Applied Ecology, 42, 201–207.

Gomez, C.M., Perez-Blanco, C.D., Batalla, R.J. 2014. Tradeoffs in river restoration: Flushing flows vs. hydropower generation in the Lower Ebro River, Spain. Journal of Hydrology, 518, 130–139.

Gore, J.A., Layzer, J.B., and Mead, J. 2001. Macroinvertebrate instream flow studies after 20 years: a role in stream management and restoration. River Research and Applications, 17(4–5), 527–542.

Hardy, T., K. Kollaus, K. Tolman, T. Heard, and M. Howard. 2016. Ecohydraulics in applied river restoration: A case study in the San Marcos River, Texas, USA. Journal of Applied Water Engineering and Research, 4, 2–10.

Hatfield, T., and J. Bruce. 2000. Predicting salmonid habitat-flow relationships for streams from western North America. North American Journal of Fisheries Management, 20, 1005–1015.

Hockendorff, S., Tonkin, J.D., Haase, P., Bunzel-Druke, M., Zimball, O., et al. 2017. Characterizing fish responses to a river restoration over 21 years based on species’ traits. Conservation Biology, 31(5), 1098–1108.

Jahnig, S.C., Lorenz, A.W., Herind, D., Antons, C., Sundermann, A. 2011. River restoration success: a question of perception. Ecological Applications, 21(6), 2007–2015.

Johnson, M.F., Thorne, C.R., Castro, J.M., Kondfolf, G.M., Mazzacano, C.S., et al. 2019. Biomic river restoration: A new focus for river management. River Research and Applications, https://doi.org/10.1002/rra.3529.

Lee, J.S. 2012. Measuring the economic benefits of the Youngsan River Restoration Project in Kwangju, Korea, using contingent valuation, Water International, 37(7), 859–870.

Kammel, L.E., Pasternack, G.B., Massa, D.A., and bratovich, P.M. 2016. Near-census ecohydraulic bioverification of Oncorhynchus mykiss spawning microhabitat preferences. Journal of Ecohydraulics, doi: doi: 10.1080/24705357.2016.1237264.

Kinzel, P.J., Legleiter, C.J., and Nelson, J.M. 2012. Mapping River Bathymetry with a SmallFootprint Green LiDAR: Applications and Challenges. Journal of the American Water Resources Association, 1–22.

Kondolf, G.M., Anderson, S., Lave, R., Pagano, L., Merenlender, A., et al. 2007. Two Decades of River Restoration in California:What Can We Learn?. Restoration Ecology, 15(3), 516–523.

Kondolf, G.M., Angermeier, P.L., Cummins, K., Dunne, T., Healey, M., et al. 2008. Projecting Cumulative Benefits of Multiple River Restoration Projects: An Example from the Sacramento-San Joaquin RiverSystem in California. Environmental Management, 42, 933–945.

Kondolf, G.M., Loire, R., Piegay, H., and Malavoi, J. 2019. Dams and channel morphology. Environmental Flow Assessment: Methods and Applications, 8, https://doi.org/10.1002/9781119217374.ch8.

Lane, B.A., Sandoval-Solis, S., Stein, E.D., Yarnell, S.H., Pasternack, G.B. and Dahlke, H.E. 2018. Beyond metrics? The role of hydrologic baseline archetypes in environmental water management. Environmental Management, 62, 678–693.

Lane-Miller, C.C., Wheeler, S., Bjornlund, H., and Connor, J. 2013. Acquiring water for the environment: lessons from natural resources management. Journal of Environmental Policy and Planning, 15(4), 513–532.

Markus, B., and Michael, G. 2012. Willingness-to-pay for river restoration: differences across time and scenarios. Environmental Economics and Policy Studies, 14(3), 241–260.

Miller, J.R., and Kochel, R.C. 2012. Use and performance of in-stream structures for river restoration: a case study from North Carolina. Environmental Earth Science, 68, 1563–1574.

Moir, H. J. and Pasternack, G. B. 2008. Relationships between mesoscale morphological units, stream hydraulics and Chinook salmon (Oncorhynchus tshawytscha) spawning habitat on the Lower Yuba River, California. Geomorphology, 100, 527–548.

Morandi, B., Piegay, H., Lamouroux, N., and Vaudor, L. 2014. How is success or failure in river restoration projects evaluated? Feedback from French restoration projects. Journal of Environmental Management, 137, 179–189.

Palmer, M.A., and Ruhi, A. 2019. Linkages between flow regime, biota, and ecosystem processes: Implications for river restoration. Science, 365(6459).

Palmer, M.A., Bernhardt, E.S., Allan, J.D., Lake, P.S., Alexander, G., et al. 2005. Standards for ecologically successful river restoration. Journal of Applied Ecology, 42(2), 208–217.

Parasiewicz, P. 2001. MesoHABSIM: A concept for application of instream flow models in river restoration planning. Fisheries, 26(9), 6–13.

Pasternack, G. B., Baig, D., Webber, M., and Brown, R. 2018. Hierarchically nested river landform sequences. Part 1: Theory. Earth Surface Processes and Landforms. DOI: 10.1002/esp.4411.

Poff. N.L., Allan, J.D., Bain, M.B., Karr, J.R., Prestegaard, K.L., et al. 1997. The natural flow regime: A paradigm for river conservation and restoration. BioScience, 47(11), 769–784.

Poff, N.L., Allan, J.D., Palmer, M.A., Hart, D.D., and Richter, B.D. 2003. River flows and water wars: Emerging science for environmental decision making. Biological Sciences Faculty Publications, 223.

Poff, N.L, and Julie, K.H. 2010. Ecological responses to altered flow regimes: A literature review to inform the science and management of environmental flows. Freshwater Biology, 55(1), 194–205.

Pompeu, P.S., Agostinho, A.A., Pelicice, F.M. 2011. Existing and future challenges: The concept of successful fish passage in South America. River Research and Applications, 28(4), 504–512.

Rice, S. P., S. Little, P. J. Wood, H. J. Moir, and D. Vericat. 2010. The relative contributions of ecology and hydraulics to ecohydraulics. River Research and Applications, 26, 1–4.

Shenton, W., N. R. Bond, J. D. L. Yen, and R. MacNally. 2012. Putting the ‘ecology’ into environmental flows: Ecological dynamics and demographic modelling. Environmental Management, 50, 1–10.

Statzner, B., J. A. Gore, and V. H. Resh. 1988. Hydraulic stream ecology: Observed patterns and potential applications. Journal of the North American Benthological Society, 7, 307–360.

Sundermann, A., Stoll, S., and Haase, P. 2011. River restoration success depends on the species pool of the immediate surroundings. Ecological Applications, 21(6), 1962–1971.

Thomson, J.R., Taylor, M.P., Fryirs, K.A., Brierley, G.J., 2001. A geomorphological framework for river characterization and habitat assessment. Aquatic Conservation-Marine and Freshwater Ecosystems, 11(5), 373–389.

Whipple, A.A., and Vier, J.H. 2019. Coupling landscapes and river flows to restore highly modified rivers. Water Resources Research, 55(6), 4512–4532.

Wohl, E., Angermeir, P.L., Bledsoe, B., Kondolf, M., MacDonnell, L., et al. 2005. River restoration. Water Resources Research, 41, W10301.

Wohl, E., Lane, S.N., and Wilcox, A.C. 2015. The science and practice of river restoration. Water Resource Research, 51(8), 5974–5997.

Woolsey, S., Capelli, F., Gonser, T., Hoehn, E., Hostmann, M., et al. 2007. A strategy to assess river restoration. Freshwater Biology, 52, 752–769.

Yarnell, S.M., Petts, G.E., Schmidt, J.C., Whipple, A.A., Beller, E.E., et al. 2015. Function flows in modified riverscapes: Hydrographs, habitats and opportunities. BioScience, 20, 1–10.

Yoshimura, C., Omura, T., Furumai, H., and Tockner, K. 2005. Present state of rivers and streams in Japan. River Research and Applications, 21(2–3), 93–112.