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The watershed-continuum model (WCM) describes fluvial-riparian ecosystems (FREs) as dynamic reach-based ecohydrogeological riverine landscapes linking aquatic, riparian, and upland domains within watersheds. FRE domains include aquatic (channels, hyporheic zones, springs, other groundwater zones and in-channel lakes), riparian, and adjacent upland zones, all of which can interact spatio-temporally. Occupying only a minute proportion of the terrestrial surface, FREs contain and process only a tiny fraction of the Earth’s freshwater, but often are highly productive, flood-disturbed, and ecologically interactive, supporting diverse, densely-packed biotic assemblages and socio-cultural resource uses and functions. FRE biodiversity is influenced by hydrogeomorphology, ecotonal transitions, and shifting habitat mosaics across stage elevation. Thus, the WCM integrates physical, biological, and socio-cultural characteristics, elements, and processes of FREs. Here, we summarize and illustrate the WCM, integrating diverse physical and ecological conceptual models to describe natural (unmanipulated) FRE dynamics. We integrate key processes affecting FRE forms and functions, and illustrate reach-based organization across temporal and spatial scales. Such a holistic approach into natural FRE structure and functions provides a baseline against which to measure and calibrate ecosystem alteration, management, and rehabilitation potential. Integration of groundwater, fluvial, and lacustrine ecological interactions within entire basins supports long-term, seasonally-based sustainable river management, which has never been more urgently needed.
*Address all correspondence to: larry@springstewardship.org
1. Introduction
Fluvial-riparian ecosystems (FREs) are watershed- (catchment-, drainage area-) based riverine landscape systems that integrate aquatic, riparian, and upland domains within watersheds, linking physical, biological, and cultural-economic processes [1, 2, 3]. From the context of a system FREs consist of “… a structured set of objects and/or attributes… (,) components or variables …that exhibit discernible relationships with one another and operate together as a complex whole …” ([4], 1–2; [5, 6, 7]). FREs include all sources of water that contribute to the basin’s riverine ecosystem, including springs, surface runoff, lakes, and atmospheric sources such as humidity and fog. Only an average of 2120 km3 (0.0002 percent) of the world’s water exists in river systems at any given time [8] (Figure 1). But while rivers process only a tiny fraction of the Earth’s fresh water and occupy only a minute proportion of the Earth’s terrestrial surface, FREs are highly productive and ecologically interactive, often supporting complex landforms and diverse, densely packed biotic assemblages that change across fine to coarse spatial and temporal scales [9, 10] and burgeoning human populations. FRE physical and biological characteristics and processes among aquatic and riparian domains step, intergrade, and may interact through reaches within the watershed, from the headwaters to the terminus in an endorheic basin or the sea, and can extend far out into the submarine environment (e.g., [11]; Figure 1). Physically, FREs are “complex adaptive process–response system(s) with …morphological system(s) of channels, floodplains, hillslopes, deltas, … and cascading system(s) of …water and sediment” [12].
Figure 1.
Surface hydrological cycle and fluvial-riparian landscape within the watershed. Numbers represent the percent of freshwater storage 6 (redrawn from [1]).
Within FREs, the riparian domain is a zone of “transition between the aquatic ecosystem and the adjacent terrestrial ecosystem [13]. Riparian zones function as filters that reduce the impacts of flooding and surface runoff, as habitats that support vegetation, fish, and wildlife populations and habitat, and often provide critically important ecosystem goods and services [13, 14, 15, 16, 17, 18, 19]. Elevated FRE biodiversity is linked to, and influenced by factors including tectonics, geology, climate, hydrology, geomorphology, and latitude, in the context of shifting habitat mosaic heterogeneity and ecotonal dynamics [20, 21, 22, 23]. Human reliance on FREs, and our species’ evolutionary history and modern demography clearly demonstrate that reliance. As human domination of the Earth has progressed, rivers have been subjected to a host of anthropogenic alterations, including resource extraction, groundwater withdrawal, flow diversion and regulation, water quality degradation, and introduction of non-native species. The natural dimensions and human impacts on FREs have stimulated deep interest, concern, and much basic and applied research, generating a vast literature and prompting development of a suite of interrelated, but not necessarily integrated ecohydrogeological models. Focus on particular aspects of FRE channel development, geomorphology, ecology, or sociology has sometimes diminished wholistic integration. Also, graphic representation of FRE ecology can be improved to enhance conceptualization, and improve educational outreach.
Here, we provide an overview description and illustrated summary of the watershed-continuum model (WCM) [1], which couples interdisciplinary physical and ecological conceptualization of FRE ecology. The WCM links conceptual models of fluvial spatio-temporal development and geomorphology across stream order and reaches FRE ecology, trophic energy and matter dynamics, biodiversity, and evolutionary interactions from the river’s source to its mouth. We provide a chronological analysis of major concepts in FRE ecohydrogeology (Table 1) and illustrate the WCM with an improved spatio-temporal, reach-linked conceptual diagram that integrates “bottom-up” physical factors, including geology, hydrology, geochemistry, geomorphology, sedimentology, and fluvial climate, with aquatic and riparian biotic assemblages and ecosystem structure within the watershed, and the potential for trophic cascade effects [26, 27, 36, 37, 99, 100, 101, 102]. Due to the brevity of this manuscript, we emphasize here integration of physical and ecological conceptual elements and processes in natural, unmanipulated FREs, recognizing that such an integration is needed as a basis to improve watershed stewardship.
Regional processes act across spatial scales to control FRE form and function
Table 1.
Concepts in physical and biological FRE ecology, presented in approximate chronological order of publication. Rows in gray indicate concepts that are primarily focused on physical processes, whereas unshaded rows indicate concepts that are more strongly focused on eco-biological issues.
We reserve more detailed discussions on the details of riparian and aquatic community ecology related to the WCM for subsequent summaries [1] but focus on Integration and clear depiction of FRE domain interactions among reaches and over time within the basin. We discuss understudied issues and opportunities, the resolution of which will help advance FRE ecology in the future. Such an objective is essential for sustainable management of rivers.
Natural FREs are terrestrial, dendritic surface-water flow paths transporting matter and energy both downslope and upslope through their channels within watersheds, with flow contributed by groundwater and multiple surface water, as well as atmospheric sources (Figures 1 and 2). Rivers play a disproportionately large role in geochemical cycling, biodiversity, human culture, esthetics, and socio-economic appropriation [98, 103]. Collectively, physical state variables regulate flow, hydrography, water quality, and sediment transport in a “bottom-up” ecosystem fashion, generating FRE habitat templates at a local scale and within reaches that are created by parent rock geology and geologic structure, as well as tributary influences [6, 67, 69, 98]. FREs are structured in relation to latitude, the extent of geological constraint, flooding, sediment transport, potential productivity, and many other factors. FRE aquatic and riparian habitats are colonized through both active and passive biogeographic processes and change across stage and over time through disequilibrial or successional processes [34, 46, 47, 48, 74, 75]. We present a rough chronology of advances in physical and biological conceptual models (Table 1), with additional references in the WCM [1], and illustrate this complex, “bottom-up” array of physical influences and responses on FRE structure (Figure 2).
Figure 2.
Conceptual FRE model depicting interactions among independent and dependent physical and ecological variables and processes, across stream order (zero headwaters to X- mouth) and time (T1 to TX). Reaches depicted reflect general patterns of width. Line thickness indicates strength of effect; black arrow points indicate relative impacts of tributaries of different sizes. C – Consumers, P – Productivity or producers, PAR – Photosynthetically-active radiation, PNAPP – Potential net annual primary productivity R – Respiration, WQ – Water quality. Redrawn from [1].
2.2 Physical conceptual models
Basin and consequent FRE development vary in relation to complex interactions among tectonic terrain, geologic structure, parent bedrock, and climate over time (Table 1; Figure 1). Tectonic setting and geologic history ultimately control regional aquifer and surface watershed development (e.g., [104, 105]). Large, high-order rivers develop over geologic time frames, particularly in interior continental settings. Basin integration across complex landscapes for many large rivers has largely occurred during Neogene time (<23.5 million yr.; e.g., the Amazon, Colorado, Mississippi, Nile, Rhine, Rio Grande, and Yangtze rivers) [106, 107, 108, 109, 110, 111], although some river basins are far older (e.g., [112]).
River basins exist in a continuous state of development, expanding and capturing adjacent drainage basins at both gradual and punctuated rates (e.g., through periods of relatively rapid crustal deformation or vulcanism) and achieving transverse integration through at least five mechanisms (antecedence, superposition, anteposition, spillover, and piracy) [113, 114]. River channels respond to perturbations such as flood events by moving toward an equilibrium state that, due to recurring perturbation, may never be reached [101]. Although poorly synthesized, the three main tectonic interactions (convergence, divergence, and transform) are likely to produce different groundwater and surface water basin configurations. Aquifers that source rivers in these landscapes often are assumed to be constrained by surface catchment boundaries (e.g., [115]); however, such is not always the case (e.g., [116]). Regional climate also affects landscape geomorphology, and sometimes reciprocally influences tectonic processes (e.g., [117]). With tectonism as a driver, river course integration within a basin has been analogized to the process of organic evolution, in which drainage-head erosion allows individual tributaries to “competitively” integrate increasingly larger catchment areas, ultimately securing throughflow to the terminus [118].
Drainage network complexity is related to stream order, which increases when two channels of the same magnitude meet [26, 27]. Many of the world’s major rivers arise from discrete springs, spring-fed marshes, or groundwater-fed lakes. Headwater springs function as “zero order” streams and springbrooks are commonly regarded as first-order channels. Rivers with spring-sourced baseflows include the Amazon, Colorado, Mississippi, Rhine, Volga, Murray, and many others [119]. Springs often exhibit strikingly different temperature and geochemical characteristics from those in the adjacent higher order streams into which they are confluent (e.g., [120]). Their unique water quality may influence mainstream processes, such as imprinting among larval fish [121] but see [122]. The ecological transition from headwater springs into first-order streams is highly individualistic, often occurring at a chemically and thermally discrete distance from the source [123]. The quantity and quality of riverside or in-stream springs, as well as seasonal flow changes driven by precipitation, also can affect stream channel geomorphology through travertine deposition (e.g., [124]), persistence of in-channel woody vegetation [125], and river water quality (e.g., geothermal spring influences; e.g., [126]).
Stream order increases erratically downstream, with middle-order streams often having the greatest productivity and species richness. FRE lacustrine reaches can occur at any order within a basin (Figures 1 and 2) and lake limnological processes like thermal or chemical stratification can influence downstream flow and geochemistry. The largest rivers include the Amazon and Volga, which are regarded as 12th or 13th order streams. Such large rivers buffer temporal and spatial changes in water temperature, geochemistry, and the timing of dynamic equilibration.
River drainage networks often are subdivided into segments and reaches, which function as organizational units within FREs. Reaches lie within segments and are best distinguished geomorphologically on the basis of differences in parent rock geology, shoreline erodibility, slope (gradient), and thalweg position (e.g., [127]). River segments (sensu [128]) include one or more river reaches that are collectively subject to distinctive changes in one or more ecosystem characteristics (e.g., water temperature, geochemistry, suspended sediment load, or gamma diversity in an assemblage). Such changes often are introduced by a tributary, thereby affecting downstream FRE ecology [70, 71, 79, 129, 130].
The frequency, duration, magnitude, and timing of both high and low flows are critical determinants of FRE ecology. Discharge and flood frequency and magnitude are increasingly monitored and evaluated in relation to human activities within watershed. Magilligan [131] described variation in channel boundary shear stress and unit stream power on an array of stream channels across 2- to 500-year floods. She noted three-fold order of magnitude variation in flood power through the basin due to valley width, with broad, alluvial channels in wide valleys subject to lower flood power. In contrast, she reported increased stream power in narrow valleys with constrained channels, a pattern influenced both by basin size and by local controls. She also suggested that maximum flood impacts on channel geomorphology occur at discrete points within reaches. Such focal points are likely to shift over time, suggesting that drainage evolution may occur most intensively at the local scale. Antecedent events are critically important, as prior high flows exert long-lasting impacts on FRE structure and ecology (e.g., [34, 132, 133], many others). Detection of such events through dendrochronology is becoming more frequently used, helping to determine long-term drought frequency and duration, and providing insights into adaptive strategic options for FRE management in the face of climate change (e.g., [134, 135, 136, 137, 138, 139]).
The impacts of natural, regular, short-term stage fluctuations in rivers are generally poorly known, but are of great consequence in management of rivers impounded for hydroelectric power production (e.g., [140, 141]). Natural semi-daily tidal bores are common in the lowermost reaches of low-gradient rivers that reach the sea. Daily variation in flow stage in such settings may desiccate or freeze macrophytes, macroinvertebrate habitats and eggs, or interrupt aquatic and riparian faunal feeding and other behaviors, leading to reduced or fluctuating primary and secondary consumer production. Understanding the effects of natural fluctuating flows remains a potentially important topic for future natural and regulated FRE research.
River water quality varies across lithology, latitude, elevation, humidity province, season, and stream order within basins and among reaches, and springs, lakes, and glacial melt influence river waters. Water quality characteristics can transition markedly over stream order and are important determinants of macrophyte structure and composition, and life history and feeding guild distributions of aquatic macroinvertebrates, fish, and amphibians, in turn influencing food web linkage ([141], but see [142]) and riparian groundwater quality and quantity. Surface flow geochemical and sedimentological changes occur at tributary confluences (e.g., [129, 130]), abruptly as discontinuities, or more gradually and to a lesser extent in side channels and other shallow, low-velocity shoreline habitats. Limnologically-influenced water quality dominates lake-sourced rivers, but we know of little research on natural downriver responses to such alteration. River water generally trends toward a universal quality across stream order, generating relatively similar geochemistry among the world’s major rivers at their mouths; however, the contributions and evolution of FRE water quality depend in large measure on subbasin geology and the relative contributions of tributaries (e.g., [143, 144]), as well as anthropogenic impacts.
The erosion, and deposition of bed, suspended, dissolved loads, and flotsam sediments are related to watershed geology, aquifer properties, flow dynamics, channel configuration, and other factors like glacial influences [29, 32, 33, 59, 127, 145, 146]. Cumulatively, including anthropogenic materials, the world’s rivers deposit about 20 billion mt of solids into the sea each year [147]. Rather than being a sole function of basin area, this deposition is largely the result of discharge from many thousands of small basins (<10,000 km2) with relatively high-gradient rivers that mouth directly into the seas [148]. Large rivers deposit proportionally less sediment due to subaqueous storage in deltas.
Recent research and stewardship attention in fluvial geomorphology has shifted to temporally based based spatial scales of reaches in the continuum of alluvial to constrained rivers (e.g., [10, 53, 67, 68]). Alluvial reaches often have relatively uniform bed materials and channel landform configuration, and often are closer to equilibrium than geologically constrained channels. Models of sediment deposition and erosion are diverse (reviewed in [149], among many others) and can provide adequate two-dimensional prediction of suspended sediment transport through channels with varying bed roughness, channel steepness, and sediment transport. However, most rivers have insufficient historical flow, sedimentological, and hydrographic data to permit high-precision modeling [150]. Variation in turbulence, sheer stress, transport capacity, and bed and suspended sediment loading are likely to increase channel landform and FRE habitat diversity among reaches. In comparison with constrained reaches, seasonally dynamic alluvial reaches often support broader riparian zones, with increasing filtering, storage, and processing of matter from upstream and local sources [75, 76, 151].
Many watershed factors Influence FRE functions. Upland wildfire, forest pest insect outbreaks, coarse wood debris loading, and overgrazing can affect fluvial FRE sedimentation, geomorphology, and nutrient and nutrient transport (e.g., [152, 153]). Ice-related impacts on FREs in temperate and boreal streams involve ice formation, “shoving”, and black-ice melting, resulting in severe scouring of shorelines and bed surfaces, damming of channels, uplifting/redeposition of fine to coarse substrata (including boulders and coarse woody debris) [153, 154].
Fluvial climate is influenced by global- to local-scale conditions, the latter including nocturnal cool air subsidence and upriver mountain valley wind patterns (e.g., [155, 156]). However, few meso-scale studies of river influences on basin, reach, or local microclimate have been conducted. Riparian and in-stream interception of photosynthetically-active radiation (PAR) varies temporally and by reach in canyon-bound rivers, influencing in-canyon air temperature, relative humidity, and aquatic and riparian production (e.g., [80, 81]). Although not yet studied, variation in PAR flux also may influence erosion in temperate cliff-dominated channels through increased slope failure frequency, cliff retreat, and canyon landform evolution. At local, cross-sectional scales, discharge, cliff shading, and channel aspect influence microclimate and fluvial solar energy flux, in turn influencing FRE air temperature and relative humidity [157], which has been positively associated with temperate avian species diversity [158]. Thus, along with riparian soil ecology, fluvial microclimate may contribute to the biodiversity and productivity of riparian zones.
Habitat complexity at tributary confluences increases ecological productivity and biodiversity, and sustains habitat spatio-temporal connectivity [10, 39, 65, 70, 71, 159]. Tributary impacts on mainstream water quality are greatest when flows of the former exceed those of the latter; however, differences in biota may follow the opposite pattern. Aquatic macroinvertebrate assemblages in small, spring-fed tributaries may substantially differ from those in the large, adjacent mainstream (e.g., [130]). The WCM depicts the magnitude of tributary influences as dark triangles of varying size (Figure 2), but we note that large or influential tributaries can exist and interact with the mainstream anywhere in the watershed. The process domain concept (PDC) posits that variation among such geophysical processes at the reach scale shapes channel form, disturbance responses, and ecosystem structure and dynamics [68].
Many FRE physical models have focused on particular aspects of FRE channel development, form, and function. Leopold summarized much of his research on alluvial channels, through which he was able to describe the negative relationship between sinuosity and slope [160]. He lamented [33] that no comprehensive channel model adequately encompassed the self-regulating nature of alluvial channels. Existing alluvial channel studies and models have been criticized for generating: (1) untested qualitative unifying theories; (2) empirical and statistical analyses that support focused semi-quantitative models; (3) reductionist applications of Newtonian theory; and (4) theoretical resolutions of primary flow eqs. [95]. Based on Morisawa’s [34] articulation of the dynamic equilibrium concept, Nanson and Huang proposed focus on the least action principle, as represented in alluvial channels through the trend toward maximum flow efficiency [95]. While the WCM does not presume that all FRE component models (particularly ecological models) will have clear, predictive mathematical solutions, placing these many concepts in a logical order and visually representing them is an important step forward (Table 1; Figure 2).
2.3 Ecological conceptual models
Ecosystem ecology developed through the diverse contributions of Linnaeus, von Humboldt, Mobius, Darwin, Forbes, Warming, Cowles, Elton, and many others. Tansley defined the ecosystem concept as involving interrelated physical and biological elements and processes [161], and Lindeman [24] and Odum [25] initiated analysis of trophic-dynamic aspects of ecology (Table 1). FREs are primarily driven by physical factors, generating “bottom-up” ecosystem structure, with dependent biotic composition, structure, function, and trophic interactions (Figures 2 and 3). Lower stream order FRE changes often occur in a punctuated, stepped, or reach-bounded fashion as FREs receive tributary contributions of sediment, water temperature, and nutrients. As with fluvial water quality, the ecology of higher-order streams generally changes more gradually, both spatially and over time.
Figure 3.
(a) Expanded detail of foodweb linkages in FREs, contrasting allochthonous (uplands and tributary) vs. autochthonous (mainstream) ecosystem energy inputs with aquatic vs. riparian domain interactions. Arrows indicate common energy pathways among trophic levels in the four FRE arenas. Not all interactions occur in every FRE, and other trophic interactions not depicted here may exist in some FREs. (b) Differential spatial or functional change in reach-based FRE structure and function can occur in response to watershed changes. For example, upland fire can result in sediment, ash, and nutrient loading through tributaries, processes that may diminish FRE productivity and ecological role in the watershed. Similarly, reduction in precipitation or groundwater alterations through climate change or aquifer depletion may reduce mainstream and riparian function. Redrawn from. [1].
Hutchinson emphasized lake ecosystem limnology (e.g., [28]), while Hynes described stream limnology [36, 37], including the spatial scale and groundwater influences on the watershed, but with somewhat less attention to the FRE riparian domain. Hynsian (lotic) versus Hutchinsonian (lentic) emphases created long-standing differences in interpretation of the roles of habitat and biotic factors on FRE research [162]. Nonetheless, combining these lines of inquiry initiated a plethora of subsequent integrative research on FRE ecology, which continues today.
The most prominent post-Hynesian FRE conceptual advance was the river continuum concept (RCC) [6, 45, 163]. The RCC described a river ecosystem as “…a continuum of biotic adjustments and consistent pattern of loading, transport, utilization, and storage of organic matter along the (ir) length” ([6], 130). The RCC regarded “…the entire fluvial system as a continuously integrated series of physical gradients and associated biotic adjustments as the river flows from headwater to mouth”, with “…maintenance of longitudinal, lateral, and vertical pathways for biological, hydrological, and physical processes” ([99], 9–10); see [12]. The RCC lent support many patterns observed in studies of low-medium order streams and ichthyological studies, primarily in mesic regions [159, 164, 165, 166]. However, it has been criticized for not fully recognizing the roles of: (1) fluvial discontinuities; (2) groundwater and spring sourcing [167]; (3) river-sourced lakes, lentic zones, and productivity hot spots (e.g., [65]); (4) hyporheic refugia [99, 101, 168, 169]; (5) riparian ecology, except as subservient to the aquatic domain; (6) the role of temporal scale in FRE development and function, including dynamic seasonal and interannual geomorphic perturbation and adjustment [34]; (7) ephemeral and intermittent stream FRE ecology [170]; as well as (8) its applicability to higher order streams [171, 172].
Subsequent to the RCC, many FRE syntheses have been undertaken, including comprehensive edited volumes and reviews (including but not limited to [1, 23, 41, 42, 43, 44, 66, 69, 73, 75, 91, 99, 101, 102, 151, 164, 165, 166, 167, 171, 173, 174, 175, 176, 177, 178]). Below we briefly describe some of the major biologically-based conceptual components of the WCM.
Ward clarified four dimensions of spatial and temporal scale operating in most lotic ecosystems, including (1) the “longitudinal” dimension up- and downstream through rivers; (2) across-channel, riparian-aquatic domain interactions; (3) vertical interactions with hyporheic habitats and groundwater; and (4) a broad temporal dimension [102]. Dynamic interactions among these dimensions contribute to the individuality in the character of FREs. Focusing on riparian stage zones and related to Ward’s considerations, Nilsson and Svedmark [76] recognized four major processes or characteristics interactively functioning in FREs. (a) Flow regime (hydrographic) dynamics regulate FRE ecological and geomorphological processes (including riparian succession through Connell and Slayters’ three modes – facilitation, inhibition, and tolerance [179]). (b) The channel provides a corridor for organic and organic transport, primarily downstream but also upstream via anemochorous and zoochorous dispersal of propagules (Figure 2). (c) The riparian zone functions as a filter and boundary between upland and riverine processes (e.g., [85, 180]). (d) Many have recognized the high levels of biodiversity and ecological interactivity of FREs, related to elevated productivity and disturbance and high levels of habitat heterogeneity (e.g., [38, 39, 181]).
The flood pulse concept emphasized the importance of high flow pulses to FRE ecology by regularly restructuring channel and riparian landscapes [56, 57]. Regular seasonal flooding accounts for the state of suspended (or perpetual) succession in natural riparian vegetation zones, particularly in constrained channels [35], with riparian vegetation zonation in belts parallel to the mainstream [182] and with composition controlled by physiological and life history characteristics (e.g., riparian response guilds models [83, 183]).
Advances in nearshore marine ecology patch and disturbance dynamics concepts (e.g., [184, 185, 186, 187]) contributed to Thorp and DeLong’s [70] riverine productivity model (RPM). The RPM posited that production, as well as decomposition, recruitment, and other important river processes are related to niche diversity, occurring at specific points within the channel, such as at tributary confluences, along shorelines, or in specific depositional settings. Thus, FREs function as microhabitat mosaics.
The serial discontinuity concept (SDC) initially was developed to describe the impacts of impoundments on regulated rivers [52, 53], but also by extension to the roles and impacts of natural dams that form lacustrine reaches, and affect natural FRE channel geomorphology, flow, and population dynamics, both upstream and downstream. Lacustrine reaches can occur anywhere in a basin as a result of tectonism, lava dams, slope failure, or glacier development, and natural dams may persist for evolutionarily significant durations. The SDC posits that the location and size of a dam reset and influence downstream recovery of FRE characteristics through tributary contributions of flow, water quality, and biota [129], and through link discontinuity [70, 71] and network dynamics [77, 78, 79]. Examples of natural dams include Lake Victoria in Uganda, which formed as a result of tectonic rifting and interruption of flow in the Kagera and other Nile River headwater streams; Lago de Nicaragua (L. Cocibolca) in Central America, which formed as a result of tectonic uplift in the lower Tipitapa and San Juan River basins; and many basalt dams ni the southwestern USA [188, 189, 190, 191]. Six types of slope failure dams that can affect rivers are globally recognized, ranging from relatively common single events with partial valley impoundment to rare, simultaneous impoundment of multiple valleys that create multiple natural lakes [191] (e.g., [192, 193, 194]). Ice dam failure also is a well-known phenomenon (e.g., the collapse of Pleistocene Lake Missoula [195], and fjord ice dam failures [196]). These natural impoundments and their failures change downstream channel geomorphology, water quality and flow, hydrography, stage relations, velocity, habitat quality and distribution, and FRE biogeography.
The RCC did not adequately integrate the ecology of ephemeral and intermittent FREs or groundwater-surface water interactions [101]. Colloquially known as dry washes, arroyos, or wadis, ephemeral channels are extremely abundant, comprising far more than half of the global stream channel network [170], and are becoming increasingly abundant as humans and climate change dewater rivers (e.g., [197]). Flooding releases CO2 sequestered by seasonal or erratic burial of organic matter and invertebrates, such as leaf litter or clams [198]. Benthic invertebrates that shred, graze, or collect organic debris often are generally absent or rare in ephemeral FREs, reducing decomposition rates and transferring those functions to microbial and physical molar actions when the stream floods. Terrestrially, ephemeral versus intermittent riparian zones are bordered by distinctive suites of xeroriparian (dry riparian) to mesoriparian perennial plant species that provide cover and food resources [43]. Analysis of an ephemeral stream in Pakistan revealed deeply rooted woody perennial shrubs in the channel, and a bed dominated by weeds after winter rains, with drought-resistant species occurring on terraces [199]. Aquatic productivity and trophic energetics of arid-land ephemeral streams are reduced and interrupted during dry seasons (e.g., [92]), generating temporal discontinuities of stream processes. Nonetheless, ephemeral channels commonly provide essential wildlife habitat connectivity, and function as punctuated, rapidly changing biogeochemical reactors [93], and warranting additional research.
FRE productivity (P) and disturbance (D) intensity interactively influence aquatic and riparian domain biodiversity through habitat and resource availability, organism size distributions, niche specialization, assemblage composition, competition, and other factors. P and D are related to colonization (C) and extinction (E) processes in insular biogeographic models of species richness, with high levels of P related to C, and high levels of D related to E (Figure 2) [46, 47, 48, 200, 201, 202, 203, 204, 205, 206, 207]. Riparian and shallow aquatic domain P and D, as well as the depth, velocity, and transparency of water, and soil moisture and nutrient content are generally negatively related to stage. FRE riparian biodiversity tends to be maximal at intermediate levels of P and D, at intermediate flood return frequencies, and terrace stages with the maximal “ecological hospitability” to potential colonists. Our observations suggest that this pattern appears to be reversed in the aquatic domain, where shallow shorelines are most biologically diverse; however, more research is needed to understand this FRE “mirror effect” between the two domains. Both the intermediate disturbance hypothesis and insular biogeographic theory [46, 47, 48, 207] were developed to describe the biodiversity of sessile taxa, such as plants and corals, not vagile species like many larger macroinvertebrates, fish, and other vertebrates, which often actively cue on hydrographic disturbances (e.g., flood avoidance by belostomatid giant water bugs [208], or hydrograph-cued spawning among fish species). Such relationships may result, in some cases, in FRE riparian trophic cascades in which top predators can reciprocally influence channel geomorphology at reach scales [82]. Such cascades are regularly observed in fish-dominated ecosystems and in some low-order fishless systems, but often are limited in FRE aquatic domains by bottom-up physical processes (e.g., hydrology, sediment transport, ice impacts), with average sheer stress/unit area often negatively related to stream order [131]. Nonetheless, the commonly observed phenomenon of elevated FRE biodiversity, particularly at intermediate stream orders, is at least somewhat related to these coupled gradient interactions and convergent life history strategies.
FREs receive and transmit multidimensional exchanges of ecological matter and energy subsidies in the watershed. Muehlbauer et al. [94] reported that avenues of exchange may be relatively narrow (50% of exchanges occurred within 1.5 m of the stream edge), and 10% of the exchanges occurred 0.5 km into the adjacent uplands. Exchanges are reported to disproportionally influence primary producers and predators, potentially affecting both bottom-up and trophic cascades (e.g., [209]). FRE subsidy exchange involves five spatially directional processes over time, including (a) gravity-driven downslope flow and material allochthonous transport; (b) downstream flow; (c) river-to-uplands eolian and zoochorous transport; (d) lateral and downward surface to hyporheic transport; and (e) upwelling artesian groundwater influences. In addition, downstream main channel or tributary flooding in very low gradient reaches can initiate upstream-directed flow [102, 210]. Like channel geometry, these FRE ecological processes respond dynamically and temporally to climate and other factors, moving toward, but never achieving equilibrium in form, function, boundary conditions, matter transport, or trophic energy dynamics described for channel adjustment by [1, 34, 93].
In the riparian domain, FRE riparian vegetation is “…a complex of vegetation units along the river network that is functionally related to the other components of the fluvial system and surrounding area” [211, 212], which interacts in a reach- and segment-dependent fashion with the aquatic domain and its associated processes)” [213]. Like the aquatic domain, the riparian domain is interactively influenced by regional climate (e.g., [214, 215, 216]) through direct forcing effects on channel roughness, bedform morphology, and sediment transport during peak flows and seasonally changing rainfall and snowmelt [217, 218, 219], and by drought [220, 221]. Groundwater availability also can affect channel and floodplain stability and riparian vegetation [222, 223, 224, 225, 226]; (e.g., [227]), [228]. Climate influences on FRE groundwater vary spatiotemporally but can provide recharge that affects reach and segment scales through precipitation and infiltration, with potentially strong seasonal variation, as demonstrated through isotopic studies [229, 230, 231, 232, 233, 234, 235, 236].
Coupling the calculation of the standardized precipitation index (SPI; [237]) with the standardized groundwater level index (SGI; [238, 239]) can be used to relate precipitation to groundwater recharge [240, 241]. These metrics affect FRE riparian productivity [242, 243] (e.g., [244, 245, 246, 247, 248]) through groundwater recharge in relation to river stage [249], and are affected by air temperature [250, 251, 252] and extreme precipitation events [253, 254, 255], which in turn affect stream discharge [256], groundwater recharge and availability [257, 258], and phreatic zone and riparian rooting depth [259]. Inorganic sediment transport and turbidity generally (but not always) increase with stream order, reducing downstream PAR availability and 1° through 3° aquatic production [81] and strongly influencing aquatic macroinvertebrate feeding guild structure [6] and riparian nutrient availability. Complex trophic relationships can develop in riparian zones, directly and indirectly influencing primary producer structure and composition. For example, leaf-beetles, grasshoppers, beavers, and ungulates all can strongly influence riparian vegetation composition, structure, and decomposition/soil formation [260, 261, 262].
FRE biogeography involves colonization, recruitment, and population establishment overland by volant and other highly vagile species, as well as passive dispersal through gravity, aerial drift, or zoochorous transport of propagules through both overland and dendritic stream corridors (Figure 2). Regardless of the pathway, FRE population persistence and assemblage resilience are predicated on the ability of a species to remain in, or disperse-recover their position in the watershed. Therefore, persistence of all FRE species requires some form of upstream dispersal, with eviction or extirpation the inevitable consequence of failed in situ or headwater recruitment strategies. Larval aquatic macroinvertebrates may drift downstream, while adult aquatic insects often fly or are blown upstream as aerial drift. Dragonflies, salmonids, and many other fish taxa migrate upstream to spawn, against the dominant current direction, and some fish transport larval unionid mussel larvae upstream. Migratory western North American warblers and other passerine birds intensively use arid-land riparian habitat as stop-over habitat during migration [263, 264], a pattern not strongly evident in mesic eastern North America [265]. Although front-based migration also occurs among some shorebirds, many waterbird species follow FRE corridors, particularly through complex landscapes [80, 266]. In addition, many non-volant vertebrate species follow river corridors as dendritic pathways, although terrestrial faunal movements can be thwarted by steep cliffs, perilous crossings, and anthropogenic landscape interruptions [80].
The riparian plant niche construction perspective [83] and niche-box model (NBM) [84, 183] classified guilds of riparian plants in relation to similarities among life history traits. The NBM incorporates and compares autecological elements for each plant species to improve prediction of vegetation assemblage development in relation to hydrography and riparian conditions. While successfully grouping some species, the large amount of variation in the NBM multivariate plots is a reminder that life history strategies vary tremendously among species, variance that is highly adaptive but which does not readily lend itself to simple classification systems.
Trajectories of vegetation succession through modes of facilitation, inhibition, or tolerance [179] differ temporally among reaches, between humidity provinces and in relation to stream order, fluvial hydrodynamics (disturbance frequency and across stage), geomorphic setting, grain-size distribution, depth to water table [267], and biological effects, such as mycorrhizal succession [268], selective vertebrate [260] or invertebrate herbivory [261, 262], in relation to plant diseases and the presence of some bird species [204]. Surrounding upland assemblages also strongly interact with upper riparian terrace vegetation (e.g., [182, 269]). Due to increased riparian soil water availability and regular flood disturbance, FRE riparian vegetation structure is not well represented by the upland-centric Holdridge [270] global vegetation model [1].
Fluvial-riparian ecosystems are hierarchically and dynamically influenced by physical and biotic processes that vary spatially among reaches, over stream order and time within the watershed, approaching but rarely achieving equilibrium in channel geometry, fluid and matter transport, biotic composition, and ecosystem energy dynamics and structure. A wide array of conceptual physical and ecological models has described aspects of FRE ecology and responses to natural and anthropogenic perturbations. However, most models have focused on single or a reduced suite of variables at site-specific, within-reach, alluvial or constrained channels, other watershed scales, and most often on anthropogenically-altered streams. The WCM emphasizes the importance of understanding temporal and spatial scaling across the entire basin in natural systems to provide guidance for improving FRE stewardship.
Despite much progress, a wide array of important ecohydrological processes, questions, and issues remain to be addressed or more fully integrated. Not presented in prioritized order, this list of additional research topics includes but is not limited to: (1) corresponding convergence toward dynamic equilibrium of physical and biological processes; (2) extent of self-similarity among physical and biological processes across reach and stream order, space, and time; (3) groundwater-surface water interactions and connectivity under changing climates; (4) the significance, extent, and roles of groundwater and headwater springs as zero order streams in FRE ecology [120, 271]; (5) natural inter-relationships among lentic and downstream lotic reaches; (6) ephemeral stream ecosystem ecology; (7) the limiting effects of photosynthetically-active radiation in canyon-bound streams; (8) stream microclimate ecology; (9) the interactive effects of flooding, ice, and glacial effects in boreal and high elevation streams; (10) multidirectional subsidy and gene exchange in dendritic pathways; (11) the role of plant physiology in riparian vegetation zonation; (12) the significance of corridors, barriers/filter, and refugial biogeographic effects in dendritic river ecosystems [80]; (13) stream order-driven and cross-sectional spatial impacts on biodiversity; (14) population and successional models among FRE biota across trophic levels; (15) FRE ecosystem genetics and the evolution of endemism across latitude, longitude, and among tectonic landscapes; and (16) the role of noise on FRE faunal assemblages. Adequately examining and incorporating these and other topics will more fully expand the WCM model through future research and will enhance collaborative discussion among hydrogeological, ecological, and socio-cultural disciplines [272]. Such data and integration efforts will improve understanding, modeling, and stewardship of FREs at local, regional, and global spatial and temporal scales.
FREs are complex, continually changing, and vital to life on Earth. Although informative and elegant, all FRE models remain incomplete, and even the most comprehensive FRE conceptual syntheses fall short of adequately representing these remarkable, important, and dynamic ecosystems. Furthermore, discipline-based or regional specificity has often limited the applicability of some models (e.g., [273, 274, 275, 276, 277, 278]). Here we present and illustrate a synthesis of FRE knowledge through the WCM, and suggest topics for further investigation. However, FREs cannot be readily, adequately, or usefully reduced to a single suite of equations or simple illustrations. For example, non-Judeo-Christian-Islamic cultures commonly view rivers as living entities, supporting divine spirits, and essential to cultural well-being. Integrating indigenous traditional ecological concepts and knowledge into improved stewardship has rarely been achieved. We suggest that improved comprehension of FREs may require consideration of other socio-cultural dimensions. Enhanced understanding of the complex, multidimensional inter-relationships among Physical, biological, cultural, and socio-economic elements and processes within watersheds is essential to improving FRE stewardship and sustaining ecological functions vital to nature, human life, and societal integrity.
We declare no conflicts of interest related to this manuscript.
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Written By
Lawrence E. Stevens, Raymond R. Johnson and Christopher Estes
Reviewed: 19 August 2022Published: 13 October 2022
Open access peer-reviewed chapter
