Rivers around the world are being regulated by dams to accommodate the needs of a rapidly growing global population. These regulatory efforts usually oppose the natural tendency of rivers to flood, move sediment, and migrate. Although an economic benefit, river regulation has come at unforeseen and unevaluated cumulative ecological costs. Historic and contemporary approaches to remedy environmental losses have largely ignored hydrologic, geomorphic, and biotic processes that form and maintain healthy alluvial river ecosystems. Several commonly known concepts that govern how alluvial channels work have been compiled into a set of “attributes” for alluvial river integrity. These attributes provide a minimum checklist of critical geomorphic and ecological processes derived from field observation and experimentation, a set of hypotheses to chart and evaluate strategies for restoring and preserving alluvial river ecosystems. They can guide how to ( i) restore alluvial processes below an existing dam without necessarily resorting to extreme measures such as demolishing one, and ( ii) preserve alluvial river integrity below proposed dams.
Once altered by dam construction, a regulated alluvial river will never function as before. But a scaled-down morphology could retain much of a river's original integrity if key processes addressed in the attributes are explicitly provided.
Although such a restoration strategy is an experiment, it may be the most practical solution for recovering regulated alluvial river ecosystems and the species that inhabit them. Preservation or restoration of the alluvial river attributes is a logical policy direction for river management in the future. Since the 1990s, the physical and environmental consequences of river alteration and management have been openly questioned.
Continued increases in flood losses, both financial and human, and the unanticipated and unwanted results of dams and channel straightening, invite reevaluation of river management. Reevaluation has even led to removing existing dams (e.g., Butte and Clear creeks in California, Elwha River in Washington), as well as implementing experimental releases of high flows (, ). Historically, river policymakers and resource managers have been less attentive to a growing body of experience, experiment, and theory concerning geomorphic processes that form and maintain alluvial river ecosystems. There are several commonly known concepts that govern how healthy alluvial channels work that we have compiled as attributes of alluvial river integrity.
These attributes can guide how to ( i) restore alluvial processes downstream of an existing dam without necessarily resorting to extreme measures such as demolishing one, and ( ii) preserve alluvial river integrity below proposed dams. This set of attributes is not a classification system or a substitute for individual study and observation on a river. It provides a minimum checklist of critical geomorphic and ecological processes derived from field observation and experimentation, a set of hypotheses to chart and evaluate strategies for restoring and preserving alluvial river ecosystems. At the ever-present risk of oversimplification, the attributes also can help policymakers appreciate many of the complex requirements of alluvial river ecosystems. Alluvial river ecosystems persist through a complex, interacting array of physical and biological processes.
For any impetus imposed on the river ecosystem (e.g., a recommended flow release), we should expect a response (e.g., scouring sand from a pool). The significance of an impetus will depend on an appropriate threshold beyond which a specific response is expected. A process, therefore, is comprised of an impetus and an expected response. To use the alluvial river attributes as guidelines for recovering or preserving critical processes, one must consider how the magnitude, duration, frequency, and timing of an impetus will exceed a threshold to produce a desired response. Rarely, however, is a single impetus imposed on a river ecosystem associated with a single response. Floods are primary impetuses for all alluvial river morphology. An increase in discharge may initiate bed surface movement and bank erosion, once the force exerted by the flood event (the impetus) has passed some threshold for movement or erosion.
This threshold may require a specific flow magnitude and duration before producing a significant morphological response. The timing and frequency of the flood also may have profound effects on a species or a population. Mobilizing sand from a pool in January may smother salmon eggs incubating in the downstream riffle. The impetus, therefore, cannot be prescribed as a simple measure of force, nor can the total reaction be as succinctly quantified or even fully anticipated. It is with this backdrop of uncertainty that the attributes were compiled. The Alluvial River Attributes The alluvial river attributes can help river managers identify desired processes, then help prescribe necessary impetuses based on useful empirical relationships and thresholds developed by river geomorphologists and ecologists.
All of the concepts deriving the alluvial attributes have been described among a wide range of professional journals, technical books, and agency reports (reviewed in ref. ), but their compilation has not been previously published.
They may not apply equally to all alluvial river ecosystems. Some rivers may not be capable of achieving certain attributes because of overriding constraints, e.g., a river passing through an urbanized corridor often is not free to migrate.
These constraints do not eliminate the attributes' usefulness; knowing what might remain broken should influence what can be repaired. Attribute No. The primary geomorphic and ecological unit of an alluvial river is the alternate bar sequence.
Dynamic alternating bar sequences are the basic structural underpinnings for aquatic and riparian communities in healthy alluvial river ecosystems. The fundamental building block of an alluvial river is the alternate bar unit, composed of an aggradational lobe or point bar, and a scour hole or pool (Fig. A submerged transverse bar, commonly called a riffle, connects alternating point bars. An alternate bar sequence, comprised of two alternate bar units, is a meander wavelength; each wavelength is between 9 and 11 bankfull widths.
The idealized alternate bar sequence is rarely found in nature, because natural geomorphic variability (e.g., valley width contractions, bedrock exposures, etc.) perturbs the idealized channel form shown in Fig. Floods flowing through alternating bar sequences frequently rearrange the bar topography, producing diverse, high-quality aquatic and terrestrial habitat. Attribute No. Each annual hydrograph component accomplishes specific geomorphic and ecological functions. Annual hydrograph components (including winter storm events, baseflows, snowmelt peaks, and snowmelt recession limbs) collectively provide the impetus for processes that shape and sustain alluvial river ecosystems.
These components are uniquely characterized by year-to-year variation in flow magnitude, duration, frequency, and timing. Hydrograph components are seasonal patterns of daily average flow that recur from year to year. For many rivers in the western U.S., these hydrograph components include summer baseflows, rainfall- and rain-on-snow-generated floods, winter baseflows, snowmelt peak runoff, and snowmelt recession (Fig. Each annual hydrograph component can be characterized by its interannual variability in flow magnitude, duration, frequency, and timing.
A subset of all processes needed to create and sustain alluvial river ecosystems is provided by each hydrograph component. Eliminate or alter the interannual variability of the hydrograph components, and the ecosystem is invariably altered. Attribute No. The channelbed surface is frequently mobilized. Coarse alluvial channelbed surfaces are significantly mobilized by bankfull or greater floods that generally occur every 1–2 years. As streamflow rises throughout a winter storm and during peak snowmelt, a geomorphic threshold for mobilizing the channelbed surface is eventually exceeded.
This flow threshold typically occurs over a narrow range of streamflow and varies spatially, depending on the morphology, grain size, and location of sediment deposits (Fig. In general, grains on the channelbed surface are mobilized many times a year, but sometimes not at all in other years, such that, over the long-term, the streambed is mobilized on the order of once a year. The duration of channelbed mobilization is a function of the duration of the high flow, which is typically on the order of days. Attribute No. Alternate bars must be periodically scoured deeper than their coarse surface layers. Floods that exceed the threshold for scouring bed material are needed to mobilize and rejuvenate alternate bars.
Alternate bars are periodically scoured deeper than their coarse surface layer, typically by floods exceeding 5- to 10-year annual maximum flood recurrences. Scour is generally followed by redeposition, often with minimal net change in the alternating bar topography. Complex alternating bar sequences are partly created and maintained by providing the natural frequency and intensity of bed scour dependent on discharges that vary in magnitude and duration. During the rising limb of a hydrograph, after the bed surface begins to move, the rate of gravel transport rapidly increases and the bed surface begins to scour. The degree of scour can be significant, up to several feet deep.
Infrequent, wet years typically generate storms with a high magnitude and long duration; scour depth will be substantial. On the receding limb of a flood hydrograph, gravel and cobbles redeposit, often resulting in no net change in channelbed elevation after the flood. Attribute No. Fine and coarse sediment budgets are balanced. River reaches export fine and coarse sediment at rates approximately equal to sediment input rates. Although the amount and mode of sediment stored may fluctuate within a given river reach, channel-wide morphology is sustained in dynamic quasiequilibrium when averaged over many years. The magnitude and duration of high flows surpassing a flow threshold for channelbed mobility are critical for balancing the sediment budget.
Chronic channelbed aggradation and/or degradation are indicators of sediment budget imbalances. A balanced coarse sediment budget implies bedload continuity; that is, the coarser particle sizes comprising the channel bed must be transported through alternate bar sequences.
Attribute No. Alluvial channels are free to migrate.
During lateral migration, the channel erodes older flood plain and terrace deposits on the outside bend whereas it deposits sediment on the bar and flood plain of the inside bend. Although outer and inner bend processes may be caused by different hydrograph components, the long-term result is maintenance of channel width. Channel migration is one of the most important processes creating diverse aquatic and terrestrial habitats: Sediment and woody debris are delivered into the river and flood plains are rebuilt on the inside of the meander. That the stream has occupied numerous locations in its valley is evidenced by direct observations of its movement over time, and by indirect evidence obtained if one digs deeply enough into the flood plain.
Gravel and cobbles laid down by the river many years before will be found. The channel does not typically migrate during periods of low flow, but migrates during flows approaching and exceeding bankfull discharge. Shear stress on the outside of bends exceeds that necessary to erode the materials on the outside of the bank. In lower gradient reaches of alluvial rivers, migration tends to be more gradual.
Attribute No. Flood plains are frequently inundated. Flood plain inundation typically occurs every 1–2 years. Flood plain inundation attenuates flood peaks, moderates alternate bar scour, and promotes nutrient cycling. As flows increase beyond that which can be contained by the bankfull channel, water spreads across the flatter flood plain surface. The threshold for this process is the bankfull discharge.
This first threshold allows flow simply to spill out of the bankfull channel and wet the flood plain surface; a slightly larger discharge is required to transport and deposit the fine sediments that are in suspension. Flood plain inundation also moderates alternate bar scour in the mainstem channel by limiting flow depth increases within the bankfull channel during floods. As water covers the flood plain, flow velocity decreases.
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Sediment begins to settle, causing fresh deposits of fine sands and silts on the flood plain. This deposition promotes riparian vegetation regeneration and growth.
Attribute No. Large floods create and sustain a complex mainstem and flood plain morphology. Large floods—those exceeding 10- to 20-year recurrence events—reshape and/or redirect entire meander sequences, avulse mainstem channels, rejuvenate mature riparian stands to early successional stages, form and maintain side channels, scour flood plains, and perpetuate off-channel wetlands, including oxbows.
A still larger flow threshold than floodplain inundation is one that scours the flood plain. The streamflow necessary to surpass this threshold is typically many times the bankfull flow because shear stress on the vegetated flood plain surface must be high enough to cause scour. Infrequent large floods are critical for sustaining channel complexity because they change river location and morphology on a large scale and prevent riparian vegetation from dominating the river corridor.
Attribute No. Diverse riparian plant communities are sustained by the natural occurrence of annual hydrograph components.
Natural, interannual variability of hydrograph components is necessary for woody riparian plant life history strategies to perpetuate early and late successional stand structures. Native riparian plant communities characteristic of alluvial river ecosystems are adapted to, and thus sustained by, a constantly changing fluvial environment. The magnitude and duration of annual hydrograph components needed for alternate bar scour, channel migration, floodplain inundation and scour, and channel avulsion provide necessary substrate conditions for successful seedling establishment and stand development. The timing and frequency of annual hydrograph components must coincide with seasonally dependent life history requirements, such as the short window of time when riparian plants are dispersing seeds. A sustainable supply of large woody debris from the riparian zone ultimately depends on variable age classes of woody riparian vegetation and a migrating channel. Attribute No.
Groundwater in the valley bottomlands is hydraulically connected to the mainstem channel. When flood plains are inundated, a portion of surface runoff from the watershed is retained as groundwater recharge in the valley bottomlands. The river corridor is hydraulically interconnected. Groundwater in the floodplain is closely connected to mainstem flows and can be periodically recharged by mainstem flooding. Avulsed meander bends often create oxbow wetlands, which retain direct hydraulic connectivity to mainstem surface flows. The alluvial river attributes can be used to recommend flow releases and other management activities below an existing dam.
Although this strategy is being considered in other locations, we will use the Trinity River in Northern California as an example, where the recovery of Pacific salmon and steelhead trout is being linked with the overall goal of restoring an alluvial river ecosystem. The Trinity River at Lewiston The mainstem Trinity River in northern California was once an alluvial river capable of constantly reshaping its channelbed and banks. In 1963, the U.S.
Bureau of Reclamation constructed a large storage reservoir and diversion tunnel to store and divert up to 90% of the natural streamflow from the Trinity River into the Sacramento River for power generation and agricultural/municipal water supply. Historically, Trinity River daily flows varied from less than 2.8 m 3/s baseflows in dry summers to near 2,800 m 3/s floods in wet winters. Snowmelt peak runoff and its recession limb were two critical annual hydrograph components generated upstream of Lewiston (Fig. In wet years, snowmelt runoff typically peaked at 340 m 3/s or higher in late June or July, whereas in dry years the peak would only be 110 m 3/s or lower in mid-May through mid-June. Together they provided the magnitude and duration of flows needed to balance the sediment budget and accomplish a wide range of physical and biological processes. Both hydrograph components theoretically could have occurred at any time of the year and still have balanced the sediment budget. But seasonal timing of snowmelt runoff was critical to ecological processes.
Peak snowmelt runoff was an important environmental cue for juvenile salmonids to begin their migration to the Pacific Ocean. Amphibians needed snowmelt runoff to keep oxbow wetlands inundated. If the snowmelt recession limb did not extend into early June, the wetland might have dried out before amphibians could complete their aquatic life history stage. Interannual variability of timing, magnitude, and duration of snowmelt recession limbs determined whether a particular oxbow wetland could sustain an amphibian population. Successful cottonwood regeneration on freshly deposited floodplains also required specific snowmelt peaks and recession limbs to create favorable moisture conditions for seedling germination, as well as the absence of extreme winter storm events the following year to prevent seedling loss. After the dam was completed, flows were kept nearly constant at 4.2 m 3/s; river managers thought that 4.2 m 3/s would provide ideal hydraulic conditions for chinook salmon spawning.
What river managers did not foresee was that by eliminating hydrograph components they would set in motion a chain of predictable events. Seedlings, no longer scoured away by frequent winter and snowmelt floods, rapidly encroached onto the alternate bars. Prominent berms of freshly deposited sand and silt accumulated along the channel margins within the maturing dense riparian vegetation (Fig. ), effectively isolating the floodplain from the mainstem river.
High shear stresses of infrequent high flow events were then concentrated in the channel's center. The river's complex alternate bar morphology was quickly transformed into a smaller, confined rectangular channel (Fig. ) now unable to meander. Floodplains were abandoned. Cumulatively, this flume-like morphology and floodplain isolation greatly reduced habitat quantity and complexity important to numerous aquatic and riparian species.
Evolution of channel geometry and riparian vegetation in response to flow and sediment regulation from the Trinity River Division of the Central Valley Project in California, 1963–1999. Salmon populations were immediately and significantly affected.
With most of their primary spawning and rearing habitat upstream of an impassable dam, the mainstem channel below Lewiston became the primary habitat provider. When young salmon emerge from spawning gravels as fry, their immediate habitat preference is for gently sloped, low velocity, exposed cobble areas typically found along predam alternate bar margins.
In contrast, the vertical banks of the postdam channel allow excessive velocities to extend up to the banks' edges. Although the constant 4.2 m 3/s dam release temporarily accommodated spawning habitat needs, fry rearing habitat became a limiting factor to salmon production because of this rapid change in channel shape.
Was the widespread habitat loss in the Trinity River predictable? Managers who expected that spawning habitat would be preserved below the dams ignored the sediment budget ( Attribute No. Trinity and Lewiston dams prevent all bed material from passing downstream; the only sources for spawning gravels are downstream tributary inputs, minor flood plain scour, and occasional gravel introductions. The snowmelt peak and recession hydrograph components were completely eliminated ( Attribute No. 2), even though this river ecosystem had been dominated by snowmelt runoff. Of the planned flow releases greater than 4.2 m 3/s, all were well below the threshold for mobilizing the channelbed ( Attributes Nos.
3 and 4), routing bed load ( Attribute No. 5), or inundating the floodplain ( Attributes Nos. 7, 8, and 10). Consequently, seedlings escaped being scoured and encroached onto the predam alternating bars ( Attribute No. Loss of the alternate bar morphology ( Attribute No.
1) was inevitable; so was the loss of habitat created by it. Was the widespread habitat loss on the Trinity River preventable? Anadromous salmonids cannot pass upstream of Lewiston Dam, therefore their habitat will never be completely replaced unless both dams are removed.
The mainstem Trinity River below Lewiston Dam cannot be brought back to its original dimension. But a scaled-down alluvial channel morphology in equilibrium with its constrained sediment budget, reduced hydrograph components, and occasional bed material introductions could greatly restore habitat abundance and quality. A new restoration approach for the Trinity River that is guided by the alluvial attributes is in its final planning stages. An environmental impact statement/report includes this new restoration strategy, developed by the U.S.
Fish and Wildlife Service and Hoopa Valley Tribe , as one fishery restoration alternative. The management goal would be to rebuild and maintain a self-sustaining alternate bar morphology and riparian community by using the attributes as a blueprint.
Planned releases from Lewiston Dam would provide snowmelt peak and snowmelt recession hydrograph components ( Attribute No. 2) to recreate physical processes that will recover an alluvial channel morphology ( Attributes Nos. 1, 3, 4, and 6–8) and sustain off-channel wetlands ( Attribute No. The sediment budget would be balanced by releasing appropriate hydrograph components with sediment transport capacities commensurate with sediment inputs ( Attribute No.
If transport capacities exceed supply, as might occur during large flood releases in wet years, bed material would be introduced into the mainstem to compensate. Riparian berms on segments of fossilized alternating bars (in the upper 64 km) would be mechanically cleared as a precursor to reestablishing dynamic alternating bars ( Attribute No. Conclusion Society is embarking on a grand experiment. Recent dam removals are merely forerunners of a much larger task ahead. Many more dams will remain than are removed.
In practice, we must rely on the crucial assumption that native species have evolved with the natural flow regime. Violating this assumption often results in consequences that can be highly significant and difficult to reverse. The intent to recover alluvial river ecosystems below dams, as proposed for the Trinity River in northern California, will be controversial. To obtain the societal benefits of water diversion, flood control, and hydropower generation, rivers will continue to receive less flow and sediment than under unimpaired conditions. But if important attributes are provided to the greatest extent possible, alluvial river integrity can be substantially recovered. The compromise will be a smaller alluvial river; it may not recover its predam dimensions, but it would exhibit the dynamic alternate bar and floodplain morphology of the predam channel.
Adobe premiere pro wedding projects free download. Although a restoration strategy guided by the alluvial attributes is an experiment, it may be the most practical direction toward recovering regulated alluvial river ecosystems and the species that inhabit them.
Riparian vegetation on the floodplain of the, close to. These and trees show the of flooding.A floodplain or flood plain is an area of land adjacent to a or which stretches from the banks of its channel to the base of the enclosing valley walls, and which experiences during periods of high discharge.
The soils usually consist of levees, silts, and sands deposited during floods. Levees are the heaviest materials (usually pebble-size) and they are deposited first; silts and sands are finer materials. Contents.Formation Floodplains are formed when a erodes sideways as it travels downstream. When a river breaks its banks, it leaves behind layers of (silt). These gradually build up to create the floor of the plain. Floodplains generally contain unconsolidated sediments, often extending below the bed of the stream.
These are accumulations of sand, gravel, loam, silt, and/or clay, and are often important aquifers, the water drawn from them being compared to the water in the river.Geologically ancient floodplains are often represented in the landscape. These are old floodplains that remain relatively high above the present floodplain and indicate former courses of a stream.Sections of the floodplain taken by the show a great variety of material of varying coarseness, the stream bed having been scoured at one place and filled at another by currents and floods of varying swiftness, so that sometimes the deposits are of coarse gravel, sometimes of fine sand or of fine silt. It is probable that any section of such an would show deposits of a similar character.The floodplain during its formation is marked by meandering or streams, and, or pools, and is occasionally completely covered with water. When the drainage system has ceased to act or is entirely diverted for any reason, the floodplain may become a level area of great fertility, similar in appearance to the floor of an old lake.
The floodplain differs, however, because it is not altogether flat. It has a gentle slope downstream, and often, for a distance, from the side towards the center.The floodplain is the natural place for a river to dissipate its energy. Meanders form over the floodplain to slow down the flow of water and when the channel is at capacity the water spills over the floodplain where it is temporarily stored.
In terms of flood management the upper part of the floodplain (piedmont zone) is crucial as this is where the flood water control starts. Artificial canalisation of the river here will have a major impact on wider flooding. This is the basis of sustainable flood management.Ecology Floodplains can support particularly rich ecosystems, both in quantity and diversity.
Yakima River Floodplain
Forests form an ecosystem associated with floodplains, especially in. They are a category of zones or systems. A floodplain can contain 100 or even 1,000 times as many species as a river. Wetting of the floodplain soil releases an immediate surge of nutrients: those left over from the last flood, and those that result from the rapid decomposition of organic matter that has accumulated since then. Microscopic organisms thrive and larger species enter a rapid breeding cycle. Opportunistic feeders (particularly birds) move in to take advantage. The production of nutrients peaks and falls away quickly; however the surge of new growth endures for some time.
This makes floodplains particularly valuable for. River flow rates are undergoing change following suit with climate change. This change is a threat to the riparian zones and other floodplain forests. These forests have over time synced their seedling deposits after the spring peaks in flow to best take advantage of the nutrient rich soil generated by peak flow. Interaction with society.
See also:Historically, many towns have been built on floodplains, where they are highly susceptible to flooding, for a number of reasons:. access to fresh water;. the fertility of floodplain land for farming;.
cheap transportation, via rivers and railroads, which often followed rivers;. ease of development of flat landExcluding and, some of the worst natural disasters in history (measured by fatalities) have been river floods, particularly in the in China – see.
The worst of these, and the (excluding famine and epidemics) were the, estimated to have killed millions. This had been preceded by the, which killed around one million people, and is the second-worst natural disaster in history.The extent of floodplain inundation depends in part on the flood magnitude, defined by the.In the United States the manages the (NFIP). The NFIP offers insurance to properties located within a flood prone area, as defined by the (FIRM), which depicts various flood risks for a community. The FIRM typically focuses on delineation of the 100-year flood inundation area, also known within the NFIP as the Special Flood Hazard Area.Where a detailed study of a waterway has been done, the 100-year floodplain will also include the floodway, the critical portion of the floodplain which includes the and any adjacent areas that must be kept free of encroachments that might block flood flows or restrict storage of flood waters. Another commonly encountered term is the Special Flood Hazard Area, which is any area subject to inundation by the 100-year flood. A problem is that any alteration of the watershed upstream of the point in question can potentially affect the ability of the watershed to handle water, and thus potentially affects the levels of the periodic floods.
A large shopping center and parking lot, for example, may raise the levels of the 5-year, 100-year, and other floods, but the maps are rarely adjusted, and are frequently rendered obsolete by subsequent development.In order for flood-prone property to qualify for government-subsidized insurance, a local community must adopt an ordinance that protects the floodway and requires that new residential structures built in Special Flood Hazard Areas be elevated to at least the level of the 100-year flood. Commercial structures can be elevated or flood proofed to or above this level.
In some areas without detailed study information, structures may be required to be elevated to at least two feet above the surrounding grade. Many State and local governments have, in addition, adopted floodplain construction regulations which are more restrictive than those mandated by the NFIP.
The US government also sponsors flood hazard mitigation efforts to reduce flood impacts. 's Hazard Mitigation Program is one funding source for mitigation projects. A number of whole towns such as, have been completely relocated to remove them from the floodplain.
Other smaller-scale mitigation efforts include acquiring and demolishing flood-prone buildings or flood-proofing them.In some tropical floodplain areas such as the of, annual flooding events are a natural part of the local ecology and rural economy, allowing for the raising of crops through. S., 2004, Encyclopedia of Geomorphology, vol. Routledge, New York. Rood, Stewart B.; Pan, Jason; Gill, Karen M.; Franks, Carmen G.; Samuelson, Glenda M.; Shepherd, Anita (2008-02-01). 'Declining summer flows of Rocky Mountain rivers: Changing seasonal hydrology and probable impacts on floodplain forests'. Journal of Hydrology.
349 (3–4): 397–410. LII / Legal Information Institute. LII / Legal Information Institute.Sources. Powell, W. Identifying Land Use/Land Cover (LULC) Using (NAIP) Data as a Hydrologic Model Input for Local Flood Plain Management. Applied Research Project, Texas State University. This article incorporates text from a publication now in the: Chisholm, Hugh, ed.
Cambridge University Press.External links Media related to at Wikimedia Commons.
Floods can be inconvenient. Large floods can be downright disastrous.Small, regular floods that inundate riverside floodplains are essential to a river’s health, and provide a wide variety of benefits to wildlife, fish and people.When we manage rivers wisely, we can keep communities safe and enjoy all of the benefits healthy rivers provide.Restoring floodplains to give rivers more room to accommodate large floods is the best way to keep communities safe. Giving rivers more room provides a number of other benefits including clean water; open space for agriculture, recreation and trails, and habitat for fish and wildlife.and floods are a natural occurrence on rivers.
Small floods are very important to the health of a river and the land around it. They nurture life in and around rivers. The fish, wildlife and plants that live in or along a river, or floodplain, often need floods to survive and reproduce.During big floods, healthy floodplains benefit communities by slowing and spreading dangerous flood waters that would otherwise flood riverside communities, harming people and property. Healthy floodplains are nature’s flood protection.Giving rivers room to move is our best protection against floods and is a great way to help keep rivers healthy. FloodsA flood happens when heavy rains or melting snow cause a river to rise and flow over its banks. Columbia, SC, Riverfront and washed out roads during 2015 flood eventHeavy rains over a short period of time can suddenly cause a flood.
Or, lighter rains falling over many days or weeks build up, eventually causing the river to overflow.Natural rivers have small floods nearly every year, usually during the rainy season. During these seasonal floods, water spreads over riverside floodplains and creates seasonal wetlands that provide critical habitat for fish and birds.
Some rivers completely transform between rainy and dry seasons.These small, frequent floods are an integral part of a healthy river. Many people think that all floods are bad. Small floods are, in fact, essential to the health of rivers. Floods benefit communities and natureFloods allow a river’s water to reach more areas above and below ground. This water can be stored and used by nature and people. They also filter pollutants out of rivers and nourishing lands to support ecosystems and fertile areas for farming.Flooding creates islands and channels and other habitat that are home to fish, birds, and other wildlife. And while they do that, floods also help flush out invasive plants and animals, benefitting native species.The reproductive cycle of many species relies on flooding to start. For example, a number of fish, such as the sturgeon, and plants such as the willow tree.Very large floods occur much less often.
They cause the river to spread out across a larger area, threatening homes and businesses constructed in the floodplain. FloodplainsThe low-lying land on either side of a river’s banks makes up a river’s floodplains. Delaware River Valley as seen from the Sussex County line after several days of rain.Floodplains are actually a part of the river. They nurture life and naturally protect us from floods.Floodplains provide many important benefits for people and nature, including: Storing and slowing floodwatersWhen a river floods, water spreads across the floodplain and slows down. Without floodplains, rivers would rise and move faster, just as water from a hose moves faster when you hold your finger over part of the opening.
Improving water qualityFloodplains act as natural filters, absorbing harmful chemicals and other pollution, making rivers healthier for drinking and swimming, and for plants and animals. Safeguarding people and propertyIf floodplains are connected to rivers, they can hold water when floods cause a river’s banks to overflow. This can help prevent floodwaters from reaching homes and businesses. They are our first and best defense against flood damage. Creating fertile soil for cropsRivers deposit sediment and nutrients in floodplains, making them very productive areas for growing crops. Nurturing lifeFloodplains are a productive environment for plants and wildlife and serve as nurseries for many species of fish.
They provide vital habitat and are important for maintaining the web of life. Providing recreationThese are ideal places for hiking, paddling, fishing, exercising, and connecting with the beauty of nature. Recharging groundwaterDuring floods, water can replenish groundwater supplies. Capturing flood water during wet years is one of the best ways to ensure adequate groundwater during droughts.For many decades, communities have built structures such as dams and levees (linear earthen berms built alongside a river) to keep flood water from damaging homes and businesses built on floodplains.
These man-made structures age and are not always effective against the largest floods. On top of that, these structures disconnect rivers from their floodplains, causing harm to wildlife and fish and eliminating any benefits that natural floodplains provide to people and communities.
Engineered Structures Are Often A Problem, Not the SolutionLarge, infrequent floods can take lives and destroy property, which is why humans have tried so hard to “control” them with levees and dams. Water breaching levee along the Mississippi RiverUnfortunately, most people don’t realize how harmful engineered structures can be. These structures can be overwhelmed by large floods.When they break or overtop they release massive amounts of water all at once, threatening lives, destroying homes and businesses and costing Americans millions of dollars. Levees and Floodwalls Can and do failSometimes floods are bigger than expected and can overtop levees or floodwalls. If a river floods for a long time water can damage a levee.
Aging dams can be overwhelmed, releasing water with deadly force. Even using the latest techniques and design, structures can and do fail, and the consequences can be devastating. Engineered Structures Make floods worseLevees and floodwalls unnaturally keep rivers within a narrow channel. This causes water to rise higher and flow faster than it would normally. This leads to more powerful and rapid flooding downstream, or creates a bottleneck which causes flooding upstream. Floodwalls Lure people into harm’s wayEngineered structures can provide a false sense of security about living in a floodplain. People assume a levee will always protect them and are not always aware of how dangerous it can be.
Levees Destroy natural flood protectionLevees separate the river from its floodplain, starving the floodplain of water. This reduces the health of floodplain ecosystems and reduces their ability to hold water during floods.
Dams and other structures block the flow of sediment and nutrients to areas downstream, which need them to support life.Giving rivers more room to accommodate large floods is the best way to keep communities safe. Whenever possible, floodplain restoration should be a go-to solution. Reconnecting Rivers to FloodplainsAcross the country, communities are realizing that natural floodplains are important to the health, safety, and prosperity of their citizens. Delaware River, Pike County–Sussex County line, within the Delaware Water Gap National Recreation Area.Reconnecting and restoring floodplains results in less flood damage.Floodplains also provide clean and abundant water supplies, places to recreate, and healthy habitat for fish and wildlife.American Rivers is helping communities reconnect rivers to their floodplains by promoting projects that reconnect rivers to their floodplains.
Communities can restore floodplains by: Setting back or notching leveesMoving a levee further back from a river can provide more room for a river to accommodate flood waters while still protecting vital infrastructure. If there is no infrastructure you can remove the entire levee or put holes in it to allow water to access the floodplain during a flood. Levee setbacks have been used on the Yakima River, Missouri River, Maquoketa River and many others in order to accommodate flood water and keep people safe. Repairing incised channelsFloodplains can also be disconnected from rivers as a river erodes deeper and deeper into its channel.
By raising the river bed or excavating the river banks, we can recreate the ability of a river to flow onto its floodplain. Moving infrastructure out of the floodplainOne of the best ways to make sure people aren’t harmed by severe floods is to move them out of harm’s way. Moving flood-prone homes and buildings to higher ground is a life-saving action that can also be the first step to restoring floodplains to more natural habitat or recreational space. Communities such as Tacoma, Washington, Tulsa, Oklahoma, Soldier’s Grove, Wisconsin, and Cedar Falls, Iowa, have relocated businesses and homes away from areas that are at risk from floods. ReforestationEven when rivers are connected to floodplains, the habitat may be degraded or overrun with invasive species. Planting native plants can bring back the native habitat with is better able to support native fish and wildlife.
Communities like Ottawa, Illinois are restoring natural floodplain habitat as a component of their regional floodplain management strategy. Nationwide solutionsAmerican Rivers works with local, regional, and state agencies and private landowners to protect and restore rivers, floodplains and wetlands. We educate decision makers and the public about the value and importance of: Natural flood protectionWe promote federal polices and plans that recognize healthy rivers and floodplains as being our best flood protection. Vibrant ecosystemsWe help decision makers and the public understand how rivers and floodplains nurture life and provide a bounty of benefits, naturally and freely.Multiple-benefit solutions.
We seek river restoration solutions that break down silos and solve multiple water resources problems, like flood management and fish habitat.By giving rivers room and restoring healthy floodplains, we can keep communities safe and improve the health of our rivers. This reform would ensure that potential homebuyers are aware when a home has flooded repeatedly—which could require its owners to purchase flood insurance.Under this reform, certain communities with homes that flood repeatedly would have to take action to reduce the risk of future flood damage, for example by improving drainage, protecting wetlands, or helping homeowners safeguard their properties. The proposal is included in the Repeatedly Flooded Communities Preparation Act (H.R. 1558), introduced in the U.S. House of Representatives on March 16, 2017.
Rivers and Floodplains is concerned with the origin, geometry, water flow, sediment transport, erosion and deposition associated with modern alluvial rivers and floodplains, how they vary in time and space, and how this information is used to interpret deposits of ancient rivers and floodplains. There is specific reference to the types and lifestyles of organisms associated with fluvial environments, human interactions with rivers and floodplains, associated environmental and engineering concerns, as well as the economic aspects of fluvial deposits, particularly the modeling of fluvial hydrocarbon reservoirs and aquifers.
Methods of studying rivers and floodplains and their deposits are also discussed. Although basic principles are emphasized, many examples are detailed.Particular emphasis is placed on how an understanding of the nature of modern rivers and floodplains is required before any problems concerning rivers and floodplains, past or present, can be addressed rationally.Rivers and Floodplains is designed as a core text for senior undergraduate and graduate students studying modern or ancient fluvial environments, particularly in earth sciences, environmental sciences and physical geography, but also in civil and agricultural engineering.
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