Preserving floodplain rivers through function-oriented environmental flows.
A restored stretch of the Drava River, Austria, exhibiting functional morphological processes.
Floodplain rivers are biodiversity hotspots and belong to the most productive ecosystems worldwide. But they have also come under multiple pressures, including abstraction, diversion and storage of river flows. As these ecosystems are particularly sensitive to hydrological modifications, reservoirs and water abstraction pose severe threats to them.
A gravel-bed floodplain river with its main elements. The arrows show the river's three-dimensional structure, i.e., its longitudinal, vertical, and transversal floodplain gradients (Hayes et al., 2018, adapted from Hauer et al., 2016).
Environmental flow regulations are increasingly implemented to safeguard river reaches downstream of dams from environmental degradation and to maintain a defined ecological condition. E-flow management, however, does often not consider the requirements of the whole river system, but only chosen instream criteria, thereby risking that other crucial aspects are left impaired.
Indeed, therefore, Thoms and Sheldon argue that e-flow assessment must go beyond the consideration of only single elements. Instead, it must focus on central ecological processes and functions and their related pivotal hydrological drivers which are needed to sustain the whole ecosystem.
E-flow assessment currently does not include the requirements of healthy floodplains.
In order to fill this gap, a study from the University of Natural Resources and Life Sciences, Vienna, WWF Switzerland, and the University of Lisbon conducted an extensive literature search. The study outcomes were published in 2018 in the scientific journal Science of The Total Environment.
In their work, the authors pinpoint fundamental principles for viable e-flow management in modified floodplain rivers. The researchers identified key flow regime elements that determine ecological functions and processes for abiotic and biotic river elements.
FF-Flow
The literature emphasizes that the natural flow regime and its seasonal components are primary drivers for functions and processes of abiotic and biotic elements, for example, morphology, water quality, floodplain, groundwater, riparian vegetation, fish, macroinvertebrates and amphibians. Based on the liaison between central flow regime elements and associated environmental components within and adjacent to the river, Hayes et al. formulated a process-oriented functional floodplain flow (ff-flow) approach.
The conceptual curve of the functional floodplain flow (ff-flow) approach integrates ecological functions and processes (top) with principal flow regime components and their seasonality (bottom; the natural mean daily flow is depicted in grey). The presented environmental flow hydrograph (depicted in blue) does not establish intra-annual flow thresholds. Instead, it is understood to be a guide towards key aspects of the annual hydrograph and their implications for various elements of floodplain ecosystems.
This is how the authors describe the ff-flow approach depicted above:
"To ensure the maintenance of ecological functions and processes, e-flow allocations need to overlap with natural flow patterns:
Fall
The beginning of the hydrological year in fall is generally characterized by low to mean flows. The floodplain slowly falls dry, and vegetation becomes dormant. The release of flow pulses, which naturally occur due to precipitation events, promote migration and breeding of winter spawners.
Winter
During winter low flows, hyporheic upwellings facilitated by earlier high flows add to in-channel flow which safeguards the upkeep of aquatic habitat quantity and provides refugia during cold or even freezing water temperatures.
Spring
By raising water levels at the onset of spring, the ff-flow approach initiates a series of ecological functions in floodplain rivers. Flushing waste products and pollutants downstream restores water quality. The gravel riverbed is scoured from organic matter and fines, which re-establishes the water exchange between surface and hyporheic habitats, enhancing successful spawning of rheophilic fish and supporting macroinvertebrate gravel and cobble communities. Lateral water seepage replenishes water tables in the river bank and floodplain which stimulates plant growth. Emulating the snow and glacier melt and their corresponding
natural sediment load, the e-flow increases in magnitude, and side arms and heterogeneous floodplain habitats become connected to the main channel. Biota such as fish and amphibians receive ecological spawning cues as well as habitats for rearing and feeding.
Flushing flows can transport (trapped) sediment downstream and ensure that the river channel and its floodplain are maintained by redistributing fluvial sediment through erosion and sedimentation, and by resetting successional processes. For safeguarding sediment transport and morphological processes, high flows must exceed the critical shear velocity threshold to mobilize and transport various particle sizes. Pioneer habitats created by the released flood peaks at a timing and recession rate which emulate the characteristics of the falling hydrograph limb under natural conditions (as flows shift from spring flood pulses to summer low flows) are particularly vital for the establishment of floodplain and riparian plant seedlings. A natural timing and recession rate of these declining flows also allows the safe return of fish into permanent aquatic habitats and amphibians can finish their aquatic development phase before their temporal habitats fall dry.
Summer
The ff-flow also suggests recreating the typical summer dry-season flow conditions. Though these low flows may produce stressful conditions for native biota, for example, through temperature increase or diminished habitat connectivity, they also incite ecological functions such as rearing and growth of juvenile fish or the desiccation of the floodplain. The drying of seasonal floodplain ponds is a prerequisite for predator-free spawning sites for amphibians. At the same time, the effects of previous flood pulses and occasionally occurring higher flows prevent the potential negative impacts of low flows by providing soil moisture for plants or hyporheic refugia for aquatic biota.
The value of minimum e-flows and regular flooding events is widely recognized, however, few studies highlight the importance of higher seasonal flows and the role of groundwater to sustain functioning riverine floodplains. During the vegetation period, floodplain
flora requires higher flow allocations and certain biotic guilds (e.g., rheophilic fish) also depend upon hydraulic conditions established by higher flows, especially during spawning. The capacity of phreatic groundwater layers to contribute to low river flows depends upon seasonal flood pulses that recharge its aquifer. Moreover, it has been suggested that the encroachment of terrestrial vegetation into hydrologically altered river channels can be reduced through increased base flows and the release of high flow pulses."
"We conclude that the ecological principles laid out in the presented ff-flow approach provide a sound basis for establishing ecologically relevant e-flows and for guiding flow restoration in temperate floodplain rivers, if constraints are considered", says lead-author Daniel Hayes.
In summary, the ff-flow approach offers a holistic conceptual framework for e-flow assessment in floodplain river ecosystems. This approach proposes a dynamic e-flow regime, as it underlines the importance of emulating the natural flow regime with its seasonal variability, flow magnitude, frequency, event duration and rise and fall of the hydrograph. By incorporating these flow regime attributes, the researchers hypothesize that the ff-flow regime will sustain self-regenerating floodplain forests, as it fulfills their four essential requirements – regular, correctly timed flows, the establishment of regeneration sites, the provision of water table conditions, and the propagation of needed materials. Thus the ff-flow approach can safeguard the protection of temperate floodplain rivers by establishing ecologically-relevant e-flows and guiding flow restoration measures.
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