Concepts and Processes Associated with Channel-Floodplain Connectivity

By Colin Byrne


Abstract


The purpose of this paper is to introduce the reader to basic river concepts and how they relate to the processes associated with channel-floodplain interactions. In order to understand floodplain connectivity, larger river concepts must be understood. The Flood Pulse Concept may be the most important river concept when considering floodplain connectivity, producing a floodplain that can be classified as an aquatic-terrestrial transition zone. In conjunction, the characteristics of a natural flow regime and flood pulse such as magnitude, duration, timing, frequency, and rate of change are of considerable importance. A floodplain adapted to the patterns of flood pulses specific to a river system will have biota specifically adapted to survive under those conditions. Biologically, floodplains are important places within the landscape for species biodiversity. Typically at odds with a natural riparian floodplain, anthropogenic use of these land areas has been critical to societal development. With human encroachment on floodplains, riparian corridors and floodplain connectivity have been greatly altered in many places. However, floodplains are now better understood and it has been suggested that connected floodplains in a natural state provide ecosystem services such as floodwave attenuation and improved water quality. Three phases of connectivity are covered including complete disconnection, ground water connection, and surface inundation. Anthropogenic changes to climate, watersheds, and river systems have drastically changed floodplain connectivity throughout the world and have created systems stressed by hydrologic alteration of flows. In the past, restoration of floodplains has often focused on small-scale projects that may not have major long-term impacts. Environmental flows should be more effective at impacting a larger area and reintroducing a more natural flow regime to many systems. A specific case study of the impacts to the Middle Rio Grande and restoration efforts that have been implemented is included as well.

Introduction


Floodplains are an incredibly complex and important component of river systems. Leopold et al. (1964) define a floodplain as “a strip of relatively smooth land bordering a stream and overflowed at time of high water.” While that definition may be an oversimplification of a dynamic system, it provides the description that might be given by a large majority of the public. Meanwhile, state and federal officials have decided that land should be regulated as a floodplain if it is inundated by a 100-year flood event (Bhowmik and Stall, 1979). Finally, Junk et al. (1989) define a floodplain in ecological terms as areas subject to periodic inundation by rainfall, groundwater, or lateral flow from rivers or lakes resulting in a biotic community specifically adapted to the environment. Ultimately, it is clear floodplains are important in a wide variety of contexts considering the existence of definitions for laws of nature as well as societal laws.

With the ecological importance of floodplains and the societal importance of locations near reliable water sources, management problems do and will exist. As of 1977 it was estimated that floodplains occupy up to 7% of the land area of the United States, and from major flood events it is clear that both human life and property as well as ecological habitat are at risk in flood prone areas (IFMRC, 1994). The options to prevent human losses such as building bigger and stronger levees compared to the restoration options for reintroduction of natural habitat suggest a dichotomy between two approaches that may not be solved. However, in order to find a balance between ecological and societal needs, solutions must be found to accommodate both approaches.

This paper seeks to introduce readers to the basic ideas of floodplain formation as well as the importance of floodplains in terms of ecological and societal benefits. Building upon the knowledge of what a floodplain is this paper will then examine how anthropogenic actions have affected the connection of floodplains with the main channel as well as groundwater within a river system. With the understanding of the importance of floodplains and floodplain connectivity, recent restoration efforts will be covered in an attempt to describe the possibilities for future societal and ecological enhancement of river systems.

Floodplain Formation and Inundation


Floodplain formation occurs through fluvial geomorphic processes that are dependent on climate, topography, lithology, soil characteristics, and organic influences (Ritter et al., 2011). These factors will dictate how much water and sediment a river carries. It has been suggested that rivers are constantly trying to achieve dynamic equilibrium between uniform energy expenditure throughout the system and a minimum rate of work (Langbein and Leopold, 1964). As river systems are continually evolving, so are their floodplains. Floodplain surfaces are typically constructed from depositional areas of a river. The evolution of a floodplain will depend on the amount and type of sediment supplied, the availability of depositional environments, and the energy of the river system (Knighton, 1998). One method of floodplain formation occurs on inside bends of meandering rivers where velocity and shear stress are lower causing deposition on growing point bar features (Leopold et al., 1964). Sediments are also deposited on floodplain surfaces during large flow events in the form of overbank deposition (Bridge, 2003). As water exceeds the depth of the main channel, that flow will spread out onto the surrounding surface. Increasing roughness as well as the exchange of momentum between the main channel and shallower overbank flow will create slower velocities and a depositional environment (Knight and Demetriou, 1983). It is clear from these formational processes that floodplains are inherently connected to the main channel of a river.

The point at which a floodplain begins to be inundated occurs as flow exceeds the bankfull discharge at a point in the river system. Observational research has suggested that bankfull discharge has a general recurrence interval of approximately 1.5 years (Dunne and Leopold, 1978). That is, a flow event filling the entire main channel has a probability of occurring 2 out of 3 years. Although large variations in this return period can exist, flows in the 1 to 2-year return period range are typically responsible for the general size and shape of the main river channel (Leopold et al., 1964). Flows exceeding bankfull discharge will no doubt overtop the main channel and inundate the adjacent landscape.

River System Concepts


In correlation with geomorphologists who have suggested river systems tend toward dynamic energy equilibrium, ecologists have suggested that biological communities will adapt to the most probable condition of the physical system (Vannote et al., 1980). Vannote et al. (1980) described this idea as the River Continuum Concept (RCC) and also stated that downstream communities will be adapted to the energy inputs received from upstream locations (Fig. 1). Therefore, based on the RCC it is clear that biota of a floodplain would be adapted to the physical landscape created by the fluvial processes of a system as well as the energy inputs the area has been subjected to. If the system is subjected to different fluvial processes or cut off from the energy inputs needed to sustain the current biota, then that landscape will no longer be of the same character.


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Figure 1. River Continuum Concept (from FISRWG, 1998 based on Vannote et al. 1980.).


With floodplains encapsulating the range from inundation at certain times to terrestrial at others, Junk et al. (1989) introduced the Flood Pulse Concept and the idea that the floodplain is an aquatic/terrestrial transition zone (ATTZ) (Fig. 2). The ATTZ is defined as neither a lotic, lentic, nor terrestrial ecosystem, but rather it is an area subject to different water levels and a dynamic littoral zone during flood events (Junk et al. 1989). Because the ideas of the Flood Pulse Concept were conceptualized for rivers in humid climates, the concept was expanded to include rivers in a temperate climate as well as the inclusion of the flow pulse, which includes important higher flows that are below bankfull discharge (Tockner et al., 2000). These flow pulses may inundate portions of the inner channel that are typically not submerged with only base flow or may produce a rise in the groundwater level (FISRWG, 1998).


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Figure 2. Flood Pulse Concept (from FISRWG, 1998 based on Junk et al. 1989).


Although surface flows and the geomorphic principles of the channel-floodplain system describe either a submerged or water-free condition for floodplains, river-floodplain conductivity is also tied to groundwater. Tockner et al. (1999) introduce a three-phase system between channel and floodplain. Phase I of the system is complete disconnection between the flowing river and floodplain, Phase II is a groundwater connection between the two systems, and Phase III is the inundation of the floodplain with surface water (Tockner et al., 1999). It is clear that connectivity between main channel flow and floodplain has to do with more than just the overflow of the main channel. Pringle (2003) defines hydrologic connectivity as the “water-mediated transfer of matter, energy, and/or organisms within or between elements of the hydrologic cycle.” According to this definition groundwater interactions between a river and floodplain are an integral part of floodplain connectivity, which is clearly important with riparian vegetation.

All of these river system concepts are intrinsically related to the temporal discharge of a river. Poff et al. (1997) describe five important characteristics of a natural flow regime that promote ecological integrity: magnitude, frequency, duration, timing, and rate of change (Fig. 3). These variations in flow will be the ultimate constraint on the river system concepts introduced above. For example, the phase of floodplain connectivity will depend on the degree of floodplain inundation during a specific flood event, which in turn depends on the magnitude of the event. As humans have manipulated river systems and hydrologically altered the natural flow regime of a river to a flow regime more beneficial to human use, these river system concepts have been impacted as well.


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Figure 3. Characteristics of a natural flow regime (from CGI, 2011 based on Poff et al. 1997).


Functions and Benefits of Floodplains


Although floodplains have often been transformed for human use, in many areas the floodplain is still a key biological habitat. Because flood pulses only inundate the floodplain for short periods of time, the floodplain is most often associated with a terrestrial ecosystem. Both a flood pulse and a shallow groundwater table provide the necessary connection to water for many riparian plant species as well as providing habitat for birds, mammals, reptiles, amphibians, and invertebrates. Examples of complete dependence on overbank flow are the common Southwestern riparian cottonwoods and willows (FISRWG 1998). A flood pulse can also provide the main channel of a river with a large number of nutrients either in the form of decaying organic matter or small invertebrates that provide food for aquatic species (Welcomme, 1979). Meanwhile aquatic species also thrive off of a connected floodplain. Fishes often migrate towards the floodplain areas to escape high flow velocities when the river is at a high stage, in search of food in highly productive floodplain waters, or calm, protected water for spawning (Welcomme, 1979). Finally, in a healthy river system where floodplains are connected to groundwater and in turn the main channel, large amounts of nutrients can be transferred from the saturated hyporheic zone and into the main channel to improve stream productivity (Stanford and Ward, 1988).

In addition to the ecological benefits floodplain connectivity produces, there are also a number of benefits to humans referred to as ecosystem services or in this case more specifically as hydrologic services. Hydrologic services are the benefits a terrestrial ecosystem may have on freshwater including increase of a water supply, water damage mitigation, or provision of water-related cultural services (Brauman et al., 2007). One hydrologic service a healthy, connected floodplain provides is flood wave attenuation, which is summarized as the delay, reduction of magnitude, and increase in duration of a floodwave through a river system as the flood moves downstream. During flood events, surface flows on the floodplain differ greatly from those flows in the main channel of a river. It has been shown that flow velocities are considerably lower in the floodplain than in the main channel and floodplain flow will have a slowing affect on channel flow due to momentum transfer even when roughness characteristics of the channel and floodplain are the same (Knight and Demetriou, 1983). In addition, roughness values in the floodplain are typically greater than the main channel (Chow, 1959). Floodplains typically have larger numbers of irregularities, obstructions, and more vegetation than the main channel (Arcement and Schneider, 1989). The increased roughness decreases flow velocity, creates incompetent flow leading to deposition, and water storage (Knighton, 1998). All of these factors will have the effect of attenuating a floodwave and in turn decrease water damage costs downstream. A second ecosystem service that floodplains provide is improved water quality. Three primary processes occurring in floodplain areas that are important to improved water quality are interception and retention of sediment-bound contaminants, contaminant uptake by vegetation, absorption by organic and inorganic soil particles, and increased flow path length for water back to a river or stream (Large and Petts, 1996). Removing contaminants in a riparian corridor along a river will no doubt improve downstream water quality.

Human Impacts on Floodplain Connectivity


Anthropogenic changes to river systems have created a disconnect between river channels and their floodplains. Two general causes of reduction in floodplain connectivity include direct separation of the channel from the floodplain and altered watershed hydrology which in turn change the magnitude, duration, and frequency of flows reaching overbank areas (Ickes, 2005). Both of these anthropogenic changes to river systems can partially or completely sever floodplain connectivity. The complete removal of connectivity between a river and floodplain will result in the abandoned floodplain becoming a terrestrial zone, or terrace, with a much drier environment no longer subject to the flood pulse concept or at least not subject to flood events on the same recurrence intervals the ecosystem is adapted. Direct separation of the floodplain through the construction of levees is done for a number of reasons. Sequestration of the floodplain, otherwise known as using the floodplain for anthropogenic uses, is important in many river systems for commercial, residential, and most often agricultural development (Ickes et al., 2005). Levees are constructed to protect land or infrastructure from flood events and are successful for the large majority of the time. However, even with the construction of levees, fluvial systems can rarely be controlled completely and infrastructure can be overwhelmed during major storm events. This can lead to inundation of the true floodplain that may now include infrastructure that has been built on the thought to be protected floodplain (IFMRC, 1994). Meanwhile, alteration of river hydrology may include the building of an upstream dam, changes in land use or land cover, removal of water from the system for human consumption, or anthropogenic induced climate change.

With changes in floodplain connectivity from both sequestration and flow alteration, geomorphic and biological characteristics of the system are likely to change. Table 1 describes various hydrologic changes to a system and the associated geomorphic response. Meanwhile, the riparian ecosystem changes in tandem with the geomorphic changes, but are often more observed due to the charismatic nature of plants and animals. Biological impacts of disconnection between the channel and floodplain are numerous. Ward et al. (1999) found that biodiversity was greater on connected floodplains compared with disconnected floodplains. Disconnected floodplains lack the intermittent flows associated with high frequency flood events common in natural systems and therefore have a lower number of species adapted to their associated conditions (Junk et al., 1989). Lack of floodplain connectivity can create major problems in a river system and has been shown to be the main cause of considerable amounts floodplain degradation. Therefore, restoration projects must seek to reestablish the connectivity common to natural systems.


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Table 1. Anthropogenic hydrologic alteration to river systems due to river engineering (from Poff et al. 1997).


Restoration of Floodplains and Environmental Flows


Goals of floodplain restoration efforts need to be clearly defined in order to promote long lasting impacts on the system. To ensure floodplain restoration is successful Opperman et al. (2010) suggest a conceptual model made up of three elements: hydrologic connectivity between the river and floodplain, a variable flow regime ensuring flow events of different magnitudes, and a large enough geographic scale for benefits to accrue to a meaningful level. Biologically and topographically, common features of healthy floodplains include side channels, natural levees, flood basins, back-swamps, ridge and swale topography, scour channels, hummocks, and mini-basins (Pierce, 2011). Different restoration practices may involve constructing landforms or reconnecting flows to an abandoned floodplain and letting the river create the features. In either of the two instances, ensuring that the floodplain remains accessible to periodic flood events is of the utmost importance, otherwise restoration efforts will only be beneficial for short time scales.

Surface and groundwater modeling techniques are two methods to try to predict the implications of changing processes associated with floodplain connectivity. Understanding the spatial and temporal changes in a hydrologic system can help in ecological and geomorphic restoration enterprises. Surface water modeling often focuses on the hydraulics of a river system, which have important implications for attenuation of flood waves. As discussed previously, floodwave attenuation is an ecosystem service to communities along a river. Studies have shown that downstream flood wave attenuation is achieved when restoration is conducted in headwater streams (Liu et al., 2004). In addition, biological restoration focusing on riparian vegetation and geomorphic channel restoration focusing on channel geometry can also increase attenuation (Anderson et al., 2004; Woltemade and Potter, 1994). However, restoration on the scale it is often conducted can have said ecological benefits, but the benefits to flood wave attenuation are not as pronounced as it would be if the restoration took place on larger scales (Sholtes and Doyle, 2011). Boulton (2007) makes the argument that floodplain restoration efforts too often focus on surface water connectivity while it is known that three-dimensional dynamics of floodplains are critically important to healthy systems. Models have been used to show the impacts of different river morphologies on downwelling associated with rivers (Kasahara and Hill, 2007). Groundwater models are also of great importance in studies associated with riparian transpiration and predicting the effects different depths to groundwater will have on riparian plant communities (Baird et al., 2005). Although modeling can help researchers and practitioners gain insight into the conditions occurring with floodplains, it is important to remember the governing processes that are creating those conditions whether it be underground or above ground.

Because small scale restoration projects along rivers, which are commonly used to recreate past conditions in localized areas, often do not lead to system wide changes, environmental flows have been introduced as method to impact entire reaches of a river. As discussed, hydrologic alteration has drastically changed the landscape of many river systems. Environmental flows seek to restore and maintain ecologically important components of the natural flow regime so that humans can also benefit from the characteristics of a healthy stream in addition to water use (CGI, 2011). The importance of a natural flow regime has been shown in a large number of studies with fish diversity, plant species abundance and type, and macroinvertebrates all showing increases and decreases associated with increasing or decreasing flow patterns (Poff and Zimmerman, 2010). More definitively, Poff and Zimmerman (2010) state that studies have shown that flow alteration is associated with ecological change, and that the ecological change increases with increasing hydrologic change. Therefore, implementation of an environmental flow seeks to substitute the natural flow regime with an ecologically friendly man-made flow regime do decrease detrimental impacts on river communities, often specifically those associated with a flood pulse.

Floodplain Restoration Case Study – The Middle Rio Grande


As is common with the majority of rivers throughout the southwestern United States, the Rio Grande has been drastically altered from its historical natural behavior. The Albuquerque reach of the Rio Grande has seen a great reduction in the frequency of major flood events and is incised because of a number of factors, most notably the flood control structure to the north, Cochiti Dam (Richard and Julien, 2003). The dam protects the Albuquerque area from highest and most damaging discharges, but in order to do so, discharges from Cochiti Dam are limited to approximately channel capacity (~7,000 cfs) in the Albuquerque Reach (USACE, 2004). Because flows are specifically limited to main channel capacity, it is clear that at least in the Albuquerque Reach of the Rio Grande that only the most severe, local flood events will cause overbank flows. In addition to dams and levees, approximately 60,000 acres of land are irrigated between Cochiti Dam and Elephant Butte reservoir (TTEI, 2004). With diversion channels, drains, and low flow channels, it is clear to see that the Middle Rio Grande has been drastically altered from its natural state. Because flora and fauna both evolve over long periods of time to become best suited for the natural environment, such drastic hydrologic changes made over the course of such a short time scale will no doubt impair their ability to thrive.

A project to increase both human use and restore native habitat within the floodplain, or bosque, of the Middle Rio Grande was the Albuquerque Biological Park Wetland Restoration Project (USACE, 2004). Because of limitations put in place by the flood mitigation strategies of Cochiti Dam, levees, and Kellner jetty jacks, the restoration was not directly tied to the river. However, the restoration was able to promote floodplain health with the removal of the jetty jacks and invasive species (USACE, 2004). Another detrimental effect of river and watershed engineering is the estimated loss of over 90% of the wetlands once found in the floodplain regions of the Rio Grande (USACE, 2004). Interestingly, this restoration project does not directly reconnect the floodplain to more frequent flooding, although it may if an extreme event made its way through Albuquerque through the jetty jack removal. It does, however, create artificial connection through the pumping of groundwater for use in the wetland ponds. This connection to a water source provides the necessary requirements for aquatic and wetland plants to thrive while also contributing valuable zoological habitat as well. This project shows the potential for small-scale restoration projects to take place on the floodplain even when societal limitations prevent full-scale reconnection.

In order to combat the disconnection of the floodplain throughout the Rio Grande, which is a function of anthropogenic impacts on the river system, much of the focus has been through reestablishment of endangered species habitat. The Rio Grande Silvery Minnow and the Southwestern Willow Flycatcher both require connected floodplain habitat, albeit in different ways. The silvery minnow is a pelagic spawner that likely developed this method of spawning so that a shifting sand bed river such as the Rio Grande would not bury its eggs (FWS, 2007). Because of the silvery minnows spawning strategy, a channelized river with high flow velocities will effectively flush the eggs downstream and into Elephant Butte Reservoir. The availability of a connected floodplain should promote low velocity flows and backwaters, which slow the eggs and allow for protected habitat (Fig. 4). Meanwhile, the flycatcher nests in willows associated with Rio Grande floodplain. As discussed earlier, these willows are dependent on high flows to disperse seeds and germinate. Prior to anthropogenic impacts on high discharges, the large velocities had enough stream power to scour previously vegetated areas for new willow growth (TTEI, 2004). Without those powerful flows, the river floodplain lacks the disturbance necessary for the recycling of flycatcher habitat (FWS, 2002). A second necessity for the flycatcher is the availability of either surface water or groundwater to sustain the vegetation over the course of the breeding season (TTEI, 2004). Restoration plans typically seek to maintain societal water uses while restoring habitat for these two endangered species, however, it is apparent that current water use practices are not working in tandem with restoration goals.


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Figure 4. An example of a complex stretch of the Rio Grande with backwaters and low velocity channels.


Current restoration projects within the Middle Rio Grande have focused on increasing floodplain connectivity through the construction of side channels. As part of the San-Juan Chama Drinking Water Project, the Albuquerque Bernalillo County Water Utility Authority (ABCWUA) is obligated to construct habitat for the silvery minnow. One of these projects is currently underway and nearing completion southwest of the Paseo del Norte Bridge (Fig. 5). At this site a 5-acre ephemeral channel is being constructed and native cottonwoods, willows, and shrubs are being planted within 40 acres surrounding the channel (Fig. 6). The site is being constructed to begin inundating at 1500 cfs while flows of 2000 cfs will fill the entire channel. Although restoration efforts such as these habitat construction projects are clearly an attempt to reconnect a localized portion of the floodplain, the larger causes of floodplain disconnection are not being addressed. As mentioned previously, a specific restoration project such as the Paseo del Norte project may have small-scale ecological benefits but a large scale ecosystem service such as flood wave attenuation is not likely to be improved (Sholtes and Doyle, 2011). The system will still chronically suffer from the impacts of changes in flow patterns due to flood control, channelization, and anthropogenic use of river water unless environmental flows are introduced. Environmental flows have been shown to benefit channel-floodplain systems in the southwestern United States and the Albuquerque Reach of the Rio Grande has been assessed as a high-priority restoration site for environmental flows (CGI, 2011).


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Figure 5. Poster describing the restoration work on the Middle Rio Grande near Paseo del Norte Bridge.


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Figure 6. Paseo del Norte restoration project under construction.


Conclusions


Well-connected floodplains are highly dynamic systems that are important for environmental biodiversity as well as ecosystem services in the form of flood wave attenuation and improved water quality. However, it is evident that well-connected, healthy floodplains are uncommon because of human encroachment, especially in urban areas, and hydrologic alteration of the natural flow regime. The hydrologic alteration produces disconnected floodplains that are unable to sustain the biota that were previously adapted to life in the aquatic-terrestrial transitional zone. Transitions of floodplains to terraces that may have only previously occurred naturally over long periods of time or during the rarest of events, are now occurring much more rapidly and not allowing species time to adapt. In order to reestablish healthy, connected floodplains small-scale restoration is not enough. For degraded river systems, flow regimes comparable to those that existed prior to anthropogenic flow alteration must try to be reinstated if native riparian corridors are to continue to exist. Otherwise, new species that are better suited for the current conditions will thrive and geomorphic characteristics of rivers will continue to be highly modified from their more natural state.

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