Floodplains Are the Areas of Land Lateral to a River That Are Inundated with Water During

Introduction

Floodplains are the areas of land lateral to a river that are inundated with water during a time when flow has been increased such as a flooding event, during snowpack runoff, or fed through groundwater and is separated from the main channel of flow of the river. These areas provide many vital functions to the river by providing nutrients, improving water quality, and also serve as a habitat that is high in biodiversity. Floodplains have many definitions and delineations, which creates a challenge in studying these biomes that are neither entirely aquatic nor terrestrial. Floodplains had in the past been limited in definition to the area that is covered by water in a 100-year storm event. This included the area known as the floodway and the flood fringe which is farther from the main channel. This definition was aimed at defining land for development purposes and in engineering assessment. (See diagram)

The floodplain is broader in its activities and functions than just the area of land that may be covered in water during a possible 100-year flood event, and has been outlined more clearly in the paper by Junk et al as, “areas that are periodically inundated by the lateral overflow of rivers or lakes, and/or by direct precipitation or groundwater…” (Junk, 111) Floodplains have also been called the aquatic/terrestrial transition zone (ATTZ), which is the area that “… alternates between aquatic and terrestrial environments.” (Junk, 111) They are also sometimes described interchangeably with the riparian zone (insert hyperlink). However for this paper the term floodplain will be used while continuing to examine the more complete picture contained in the aquatic/terrestrial transition zone.

The actual area of a floodplain in a given river may be possible to define, however it is a dynamic changing system that can be aquatic, terrestrial, or a combination. “The environmental change from the aquatic to the terrestrial phase at a specific point in a floodplain may be as severe as the change from a lake to a desert.” (Junk, 113) This change occurs temporally and spatially like many of the other attributes of a river. “The inshore edge of the aquatic environment that traverses the (floodplain) is the ‘moving littoral’.” (Junk, 111) The littoral zone is the area submerged in water that lies between the main river channel’s limnetic zone and the adjacent floodplain. (see diagram)

Objectives

The objectives are to study the function and ecological purpose of a river’s floodplain, the changes that have occurred historically to floodplains and methods that can be employed to restore the function and connectivity of the floodplain to the river. Floodplains have many known beneficial uses and studies continue to quantify those benefits. Anthropogenic changes to rivers have severely altered and in many cases completely disconnected rivers from the adjacent historical floodplain. The impact of these changes and the loss of the floodplain function and benefit must be assessed and quantified. “The movement of water through ground and surface systems, floodplains, wetlands and watersheds is perhaps the greatest indicator of the interaction of natural processes in the environment.” (FEMA Training, Chapter 8) Restoration efforts to improve the quality of the floodplain and its connection to the river should be a priority in many river restoration efforts. Analysis and discussion of these efforts will be defined.

Floodplain Functions

There are many functions that a floodplain provides. The first and most obvious is flood wave attenuation. A flood wave is the rise and fall of the water level in a river due to a storm or sudden snow melt. A functioning floodplain will receive the water as it rises and attenuates this wave by taking the energy from the wave laterally. Rivers without the function of flood wave attenuation will carry the energy from the wave further downstream. The floodplain provides storage during a flood as well as conveyance. It reduces flow velocities and peaks and also distributes sedimentation more evenly. Floodplains store this water for use during dry periods. “One acre of floodplain land flooded one foot deep holds 330,000 gallons of water.” (FEMA)

Floodplains also provide a rich ecosystem and are part of a total functioning system with the local environment, and provide habitat that is not available elsewhere. “Fish yields and production are strongly related to the extent of accessible floodplain, whereas the main river is used as a migration route by most of the fishes.” (Junk, 110) A functioning floodplain maintains biodiversity, provides breeding and feeding grounds for a variety of species, and enhances the integrity of the ecosystem. “Many of the organisms colonizing floodplains have developed adaptations that enable them to survive during an adverse period of drought or flood and even benefit from it…” (Junk, 113)

Water quality

Floodplains are also an important factor in improving the quality of water that enters a river after a storm. The floodplain filters impurities from runoff, processes and distributes organic wastes, and also moderates the temperature of the water.

Nutrient Transfer

The transfer of materials from the floodplain to the river as well as the river to the floodplain have been studied and quantified as to its importance. “Materials are transported from the river to the floodplain (e.g. suspended sediments and nutrients) as well as from the floodplain to the river (e.g. organic detritus and algal biomass).” (Tockner, 522) Functioning and connected floodplains have an important transfer to and from the river. This transfer is necessary for the development of sustainability of many biological processes. “Floodplains are distinct because they do not depend on upstream processing inefficiencies of organic matter, although their nutrient pool is influenced by periodic lateral exchange of water and sediments with the main channel.” (Junk, 110)

“…flooding causes a perceptible impact on biota and that biota displays a defined reaction to flooding.” (Junk, 110)

Floodplain Connection

Functioning floodplains go through a process of three phases. These phases are outlined in the paper by Tockner. Each of these phases has its own function and benefit to the environment and the river. “Shifts in the strength and type of hydrological connectivity have major implications for structural and functional attributes of riverine floodplains. (Tockner, 530)

The first phase which occurs during the majority of the time is floodplain disconnection from the river. “Some lakes and swamps are isolated from the main channel for many months or even years.” (Junk, 113)

“…nutrient levels and primary productivity are low in the floodplain,…” (Tockner, 530).

Autogenic processes dominating (e.g. sedimentation of autochthonous material, nutrient uptake and grazing) as the ‘biotic interaction phase’.” (Tockner, 530)

Phase II of floodplain connection is the seepage inflow phase. “With only a subsurface connection upstream, the floodplain contributes already considerable amounts of algal biomass and DOC to the main river during phase II.” (Tockner, 531)

“With increasing water levels, the floodplain shifts from a closed to a more open system fed by nutrient-rich ground water.” (Tockner, 530)

“…define phase II as the ‘primary production phase’.” (Tockner, 532)

The third floodplain connection phase is the upstream surface connection this is also thought of as the ‘transport phase’. “The floodplain shifts from an autotrophic system during phase II to a heterotrophic system during/after flooding…” (Tockner, 532)

“…the timing of flooding and the period since the last flood are essential in explaining transport patterns.” (Tockner, 532)

Causes for Floodplain Disconnection

Many changes have been made to rivers, especially in urban areas, that have affected the connection of floodplains and disrupted the natural phases. Anthropogenic changes to the land uses along rivers, as well as the many methods employed to alter and manage the flows associated with these rivers, have created a disconnect between the river and its floodplain. “Most large rivers in the temperate zone have been greatly impacted by the construction of hydropower plants, regulation work for navigation purposes, land reclamation projects and large-scale flood control measures (e.g. Dynesius & Nilsson, 1994).

Dams (connect to dam wiki) for flood control, water storage, and occasionally hydropower have had a significant impact on floodplain biology. Dams and the reservoirs that are created as a result of dams artificially control the flow of the river, reduce flooding, and drastically alter sedimentation pathways.

Water diversions from rivers for agricultural and human uses changes the amount of water available in a river. Often the water that is withdrawn from the river from these diversions significantly decreases the amount of flow in the river. The San Juan-Chama project in New Mexico is an example of a water diversion that adds significant flows to a reach of a river that would not normally be there while detracting from another system. Water diversions from a river, whether adding flow or decreasing the flow, has an impact on the river and decreases the connectivity to the floodplain.

Rivers have been increasingly channelized as a way of controlling flooding. Channelization keeps the flow within the main river channel and reduces the amount of flow that can cross into the floodplain.

Levees have been put into place along many reaches of rivers as a way of controlling flooding. This disconnects the river from the floodplain, and eradicates an entire system that had previously been in place.

Water withdrawals

Groundwater pumping

Effects from Loss of Connectivity

Flood Pulse Concept

“The principal driving force responsible for the existence, productivity, and interactions of the major biota in river-floodplain systems is the flood pulse.” (Junk, 110)

i. Benefit of controlled flood pulse

ii. Differences from natural flood pulse

Restoration

Restoration of floodplains and reconnection to the river system is an important aspect to river restoration. The function of floodplains cannot be ignored. A focus of river restoration is “…to restore a more natural hydrograph with gradual changes in floodplain inundation and exchange processes.” (Tockner, 533)

Case Studies

Environmental Flows (link to Ryan M's Wiki)

Future Restoration for the Middle Rio Grande Flood Plain

Conclusion

Floodplains have a specific design and use as it pertains to the environment. It is part of an integrated system. “Surface water, ground water, floodplains, wetlands and other features do not function as separate and isolated components of the watershed, but rather as a single, integrated natural system.” (FEMA Training, Chapter 8) Removal or alteration of a single process has ramifications throughout the system as a whole. Floodplain connection and interaction must continue to be prioritized in river restoration.

I think your paper is well presented and follows a cohesive procession of thought. I know the Junk and Tockner papers are right on point for this topic but it may help to find some other papers to support your writing in some places. Overall, I think you’re headed in a good direction.