BENEFITS OF TREES/EFFECT ON FLOODING

"Trees reduce topsoil erosion, prevent harmful land pollutants contained in the soil from getting into our waterways, slow down water run-off, and ensure that our groundwater supplies are continually being replenished. For every 5% of tree cover added to a community, stormwater runoff is reduced by approximately 2%."
- Coder, Dr. Kim D., "Identified Benefits of Community Trees and Forests", University of Georgia, October, 1996.

"Research by the USFS shows that in a 1 inch rainstorm over 12 hours, the interception of rain by the canopy of the urban forest in Salt Lake City reduces surface runoff by about 11.3 million gallons, or 17%. These values would increase as the canopy increases."
- American Forests, "The Case For Greener Cities", 1999.


Benefits of Trees
CaseyTrees, Washington DC

Reduce Stormwater Run-Off

When land is paved over or covered, by roads and roofs for example, it is like putting “Saran Wrap” on top of the ground. Rain that would have soaked into the ground and recharged the groundwater now runs across the surface creating “stormwater run-off” which must be collected, conveyed, and discharged to a receiving body, typically rivers. As more water is running off the earth than soaking into it, more water must fit into existing pipes. These pipes are not large enough to handle the increased flows, causing flooding and overwhelming our rivers with pollutants which make our waterways unsafe for swimming and the fi sh too toxic to eat. Trees put holes in the “Saran Wrap” and allow rain to infi ltrate into the ground and recharge groundwater rather than running off the surface.

• In the Washington DC Metro area, the existing tree canopy reduces the need for retention structures by 949 million cubic feet saving the region $4.74 billion in construction costs per 30-year construction cycle.

• In the corresponding period from 1973 to 1997 where heavy tree cover decreased 64% in DC, stormwater run-off increased 34%. Consideration of trees in the City’s proposed $1.9 billion Long Term Control Plan is a significant change in the design objectives and would likewise signifi cantly reduce the City’s need for, and cost of, capital infrastructure.


http://www.fs.fed.us/ecosystemservices/watershed.shtml
FS Home / ES Home / Watershed Services

Healthy forests and wetland systems provide a host of watershed services, including water purification, ground water and surface flow regulation, erosion control, and streambank stabilization. The importance of these watershed services will only increase as water quality becomes a critical issue around the globe. Their financial value becomes particularly apparent when the costs of protecting an ecosystem for improved water quality are compared with investments in new or improved infrastructure, such as purification plants and flood control structures – in many cases it is often cheaper and more efficient to invest in ecosystem management and protection.

http://www.greenfacts.org/en/forests/toolboxes/box-6.1.htm
Influence on water regulation

The influence of forests and forest alteration on water yield and timing is complex. Where forests were the original land cover, the protective effect consists in maintaining as far as possible the ‘natural’ flow regime, which inevitably consisted of both flooding and low flows to which stream channels and associated biota were adjusted. With human intervention and occupancy, there is a need for better understanding of the forest/water interaction. With regard to floods, it is now quite clear that forests reduce stormflow peaks and delay them better than other land cover, but that this effect occurs close to a forest and diminishes further downstream in the watershed (Hamilton and King, 1983). On major rivers, headwater forests have little or no effect in reducing flood intensity in the downstream reaches (Hewlett, 1982; FAO, 2005f). But close to the protective forest, the frequent, lower intensity storms are ameliorated more than with other land covers or land uses, to the benefit of local people.

World Wide Fund for Nature
FACT SHEET
Temperate Forests
Factsheet No.35

Why do we need temperate forests?

Trees protect soil in a number of ways. Their roots open up spaces in the ground, creating a sponge effect that allows water to penetrate slowly and be stored underground. They can therefore help prevent soil being washed away by rain and rivers, and safeguard against flooding.

Forests also help produce the air we breathe and can help protect the world from the threat of global warming. While they grow, trees absorb carbon dioxide (CO2) and produce oxygen. As their growth rate slows down so too does their capacity to take up C02, although they continue to store the CO2 they have absorbed during the growth process. The microorganisms living in the forest soil also collect and store carbon. The fact that forests absorb CO2 has led to controversial schemes, which set out to use reforestation programmes to combat the threat of global warming.

*FORESTS, AIR & WATER*
Alabama Forestry Commission

Alabama's forests supply us with many benefits. Some of these are easily seen, such as wood and wildlife. Some, however, are not as noticeable, but can be even more important to us. Two of these hidden benefits are clean water and clean air.

Forests act as living sponges, catching rain water, cleaning it and slowly releasing it into Alabama s rivers and streams. The layer of leaves and twigs on the forest floor, called litter, cushions the force of raindrops and prevents soil erosion. Forest soils can absorb up to 18 inches of water from rain or snow. Tree roots hold the soil and prevent it from washing away.

The cleaning value of forests has been known for decades, and has been actively used to protect water quality near reservoirs and lakes. Studies have shown that trees planted on open cropland can reduce soil erosion from 10 tons per acre per year to less than l/2 ton per acre in 3 years.

Trees are nature's air conditioners. An acre of trees can grow about 2 tons of wood per year. This same acre removes 3 tons of carbon dioxide and produces 2 l/2 tons of oxygen per year. A person uses less than one pound of oxygen per day, or about 278 pounds per year. So every acre of forest produces enough oxygen for 18 people. At the same time, an acre of forest removes 13 tons of dust from the surrounding environment and releases 8,000 gallons of water into the air. This results in forest areas being cooler and cleaner than surrounding areas.

A Landowner’s Guide to Sustainable Forestry in Indiana
Part 5. Forests and Water
Purdue University Cooperative Extension Service

Reforestation Benefits Water Resources

Planting trees produces multiple benefits, not the least of which is the protection and improvement of water resources. Large tree planting efforts in the 1930’s through the 1960’s stabilized many eroding farm fields, stopping soil erosion and improving the water draining from those areas. Marginal or eroding cropand pastureland may be more profitably managed as forest and wildlife habitat.

Protective native forest has been cleared from many of Indiana’s stream- and riverbanks. Most of this clearing was to facilitate drainage for agriculture and to develop the rich soil in these stream and river bottoms for row crop production. Housing, commercial, and industrial development have also taken a toll on many streamside forests.

Although development is necessary, poor location decisions have unnecessarily degraded some of Indiana’s water resources. Replanting trees on stream banks and in 50- to 100-foot, or wider, buffer strips next to streams, rivers, and lakes will, in time, restore many of the benefits that native forests provide to water quality and fish habitat. River bottoms and wetlands can be very productive forestland. If you own such land and are thinking of taking it out of crop production, consider reforestation for timber production and wildlife habitat. Many incentives are available for doing so.

Avoid Clearing Forest

Since forest located anywhere on the landscape protects and improves water, it should not be cleared for other land uses if there are better alternatives. When building a new home, for instance, choose a site that is already open or on the edge of the forest instead of an interior site.

For reasons already mentioned, it is particularly important to maintain forest next to streams, rivers, lakes, and sinkholes. If you choose to clear forest for crop or livestock production, maintaining a 50- to 100- foot buffer of undisturbed forest next to these water resources will provide much of the protection desired


http://en.wikipedia.org/wiki/Deforestation
Water cycle and water resources

Trees, and plants in general, affect the water cycle significantly:

* their canopies intercept a proportion of precipitation, which is then evaporated back to the atmosphere (canopy interception);
* their litter, stems and trunks slow down surface runoff;
* their roots create macropores - large conduits - in the soil that increase infiltration of water;
* they contribute to terrestrial evaporation and reduce soil moisture via transpiration;
* their litter and other organic residue change soil properties that affect the capacity of soil to store water.

As a result, the presence or absence of trees can change the quantity of water on the surface, in the soil or groundwater, or in the atmosphere. This in turn changes erosion rates and the availability of water for either ecosystem functions or human services.

The forest may have little impact on flooding in the case of large rainfall events, which overwhelm the storage capacity of forest soil if the soils are at or close to saturation.

Soil erosion

Undisturbed forest has very low rates of soil loss, approximately 0.02 metric tons or 40 lbs per hectare. Deforestation generally increases rates of soil erosion, by increasing the amount of runoff and reducing the protection of the soil from tree litter. This can be an advantage in excessively leached tropical rain forest soils. Forestry operations themselves also increase erosion through the development of roads and the use of mechanized equipment.

China's Loess Plateau was cleared of forest millennia ago. Since then it has been eroding, creating dramatic incised valleys, and providing the sediment that gives the Yellow River its yellow color and that causes the flooding of the river in the lower reaches (hence the river's nickname 'China's sorrow')

Side Effects of Deforestation -

The Charleston Daily Mail's 1/10/06 editorial, "Science vs. popular belief," is not a new debate. In 1911, A. B. Brooks (now in the West Virginia Forestry Hall of Fame) wrote, "Forests not only produce wood . . . they hold the water of rains and melting snow and give it out gradually to the springs and regulate the flow of creeks and rivers. . ."

In 1921 the Society of American Foresters, referring to the flood of 1907, stated: "By that time, it had become increasingly obvious to both professional foresters, and many of the state's citizens, that the flooding was a direct result of the cutting of the timber."

The Wheeling Daily News printed, on March 16, 1907: "Again the Ohio River, by its conduct, forcibly reminds us of the folly of timber destruction. No other cause than devastation of the forests could have given the Ohio Valley such a deluge following the fall of a comparatively slight volume of water."

A. B. Brooks in 1911 wrote further: "Generally speaking a woodland soil absorbs more water than naked ground. The decaying leaves, the roots and stems, and the more porous nature of the upper layers of the forest soil, take up the rain and melting snow, and hold it for a time, permitting it to filter away slowly and enter the streams gradually. Sudden rushes of water down steep slopes after a rain are thus hindered, and the streams rise more slowly, flow more regularly, and seldom reach excessively low stages. When the same has been laid bare and packed by its own weight and under the unobstructed beating of raindrops, its surface hardens, its porosity is lessened, and it sheds water like a roof. The streams catch it quickly and floods follow. That is the difference between a forested and treeless region."

The U.S. Forest Service research facilities in Parsons, West Virginia and Coweeta, N.C., have published research (science) which indicates that runoff from a summer storm will be more than 9 times greater below a clear-cut than below an undisturbed forest. They have also determined that anytime the forest canopy is reduced by more than 23%, there will be a measurable increase in runoff.

In 1905 West Virginia Gov. Albert B. White declared: "The time has gone by when the man who deforests lands is a public benefactor."

JOURNAL OF FORESTRY, Vol. 56, No. 1, January 1958
[Forest] Litter does function effectively in reducing raindrop impact and subsequent erosion, in slowing overland flow and allowing more time for infiltration of water, in maintaining surface soil in condition for rapid infiltration of water, and in reducing erosion by holding the soil in place.
Louis J. METZ
Piedmont Research Center,
Southeastern Forest Experiment Station,
Forest Service, U. S. Department of Agriculture

UK Forestry Commission
The case for forestry reducing flooding

Forests are known to use more water than shorter types of vegetation. This is mainly due to the interception of rainwater by their aerodynamically rougher canopies. Interception can reduce the amount of rainfall reaching the ground by as much as 45% or more for some types of forests. A reduction of even half of this amount could therefore make a major contribution to flood control.

Another way that forests could affect floods is by their soils holding back and delaying the passage of rain water to streams and rivers. Forest soils tend to have a more open structure resulting from greater amounts of organic matter and the action of tree roots and soil fauna. They are also usually drier during summer periods due to the higher water use by forests. These conditions enhance the ability of the soil to receive and store rain water and is commonly referred to as a ‘sponge effect’.

The role of floodplain woodlands

Although most of Britain’s original floodplain woodland has been lost due to past river engineering and land reclamation works, there is good evidence to suggest that it could have an important role to play in ameliorating downstream flooding. This is based on woodland’s greater hydraulic roughness compared to other vegetation types, which acts to slow down and thus potentially reduce flood peaks.

The role of woodland in flood control: a landscape perspective
T.R. Nisbet and H. Thomas
Forest Research, UK.

Abstract

Sustainable flood management is increasingly looking to the role of catchment land use in alleviating downstream flooding. Woodland presents a number of opportunities that are dependent on its location within the landscape. One way that woodland can attenuate flooding is through the greater water use by trees.
The overall impact on the generation of flood flows, however, depends on the interaction of many factors and is most marked at the headwater level. Another way relies on the ‘sponge effect’. Improved infiltration resulting from the targeted planting of sensitive soils or the use of down-slope woodland buffers could
attenuate rapid run-off at the local scale. Finally, the greater hydraulic roughness associated with riparian and floodplain woodland can aid the retention and delay the passage of flood waters, potentially assisting downstream flood defence in larger catchments.

Conclusions

Woodland offers a number of potential opportunities for flood control. Research and experience indicates that those provided by the greater water use by trees and the forest sponge effect are largely restricted to the headwater or small catchment level. Modelling studies suggest that floodplain woodland offers the main way of ameliorating extreme flood events at the large catchment scale, although results remain to be tested in practice. Overall, there appears to be significant scope for using woodland to help reduce flood risk, as well as to provide a wide range of other environmental, social and economic benefits. However, in order to achieve these, woodland needs to be better integrated with agriculture and other land uses as part of a whole-catchment approach to sustainable flood management.


A typical Riparian forest along a stream surrounded by farmland.

CITY OF FRANKLIN COMPREHENSIVE PLAN
RECOMMENDED FOR ADOPTION BY THE PLAN COMMISSION ON:
10/22/02
RESOLUTION NO.: 02-38
ADOPTED BY THE CITY COUNCIL ON:
11/25/02
RESOLUTION NO.: 02-12

What is a Riparian Area?
Riparian areas are also known as stream-side forests. They are the wooded areas along rivers and streams. These areas are a complex ecosystem vital to the protection of stream and river water
quality. These areas include some of the richest varieties of plants and animals in most regions.

Why are Riparian Areas Important?
Land along waterways have significant ecological and aesthetic value which enhances the natural environment of a community. The presence of riparian areas also adds value to properties with
water access, as they as they are often prime locations for development.

Many communities depend upon local rivers and streams for recreation, drinking water, and natural resource areas. The loss of riparian areas along such waterways is a major cause of decreases in
water quality and loss of wildlife habitat.

How are Riparian Areas Identified?
Healthy riparian areas are typically composed of large trees, woody understory trees and shrubs, and smaller flowers, grasses, and groundcovers. Well maintained and managed riparian areas are able
to influence the physical, chemical, and biological characteristics of the stream by:
1) Providing food, shelter and natural linkages for a wide variety of plant and animal communities.
2) Shading and cooling the stream to enhance aquatic habitats.
3) Filtering sediments and pollutants, preventing them from entering the stream or waterway.
4) Stabilizing river banks and reducing bank erosion.
5) Providing flood control.

Who Regulates Riparian Areas?
In Indiana, the Indiana Department of Natural Resources has the authority to regulate riparian areas for water quality purposes. Local governments may regulate, to some extent, development or encroachment to riparian areas through planning and zoning controls.

The Natural Resource Conservation Service (NRCS) has developed Conservation Standards for Riparian Forest Buffers (Code 391). These standards are site specific and will vary depending on the size of the waterway and floodplain. Most standards address an area ranging from 35 to 150 feet on either side of the stream.

The ideal riparian area includes three zones for management in which development should be restricted. These zones, listed in sequence from the edge of the stream, are as follows:
1) Undisturbed Forest - This zone is adjacent to the stream and is ideally 15’ in width. Removal of vegetation is not permitted.
2) Managed Forest - This zone is ideally 60’ in width and harvesting of older vegetation is encouraged to support better filtering/removal of nutrients through younger, faster growing vegetation.
3) Runoff Control - This zone is ideally 20’ and may be pastured, farmed for hay or mowed for recreational purposes.


Riparian Forest Buffers
Kansas Forest Service

Benefit and Value

Riparian forests are located adjacent to streams (both perennial and intermittent), and open bodies of water. They differ from upland forests in terms of topography, soils, function, and species mix. In general, riparian forests are located on deep soils and have a more diverse species mix than the drier upland forests.

Hydrologically, these forests filter runoff and sediment from the adjacent lands, slow damaging flood waters, and recharge aquifers. As water from adjacent land flows through the riparian forest buffer, it is slowed by tree trunks, understory vegetation, and the organic litter layer.

Riparian forests trap nutrients and organic debris. As runoff from adjacent lands flows through a riparian forest, the water is slowed, allowing soil particles, nutrients, agricultural chemicals, and bacteria to settle. It is important for sediment to settle within the forest buffer to reduce sediment loads in streams and the filling of lakes and reservoirs. This settling process is especially effective at reducing phosphorus in runoff because 85 percent of available phosphates are bonded to small soil particles.

Soil bacteria and fungi break down nitrogen and organic debris into mineral nitrate forms. These nitrates are absorbed by plant roots, while other bacteria convert dissolved nitrogen into various nitrogen gasses that are returned to the atmosphere.

Maintaining streamside vegetation is an easy and cost-effective way to control nutrient problems in streams and rivers. By physically slowing runoff, riparian vegetation also increases infiltration, which stores flood water and recharges aquifers. In this way, damaging flood waters are released slowly, maintaining stream flow. Oxbow channels and adjacent wetlands also serve as overflow and storage basins during high flows.

Riparian forest buffers maintain streambank stability, especially during floods. Aerial photos taken before and after the 1993 flood in Kansas showed riparian forest buffers along 37 miles of the Kansas River were responsible for the deposition of an average of 10 feet of soil while grasslands and croplands lost an average of 78 feet and 155 feet of sediment respectively.

In Kansas, riparian forests serve as excellent wildlife habitat and support many game and nongame species of mammals, fish, amphibians, reptiles, and birds. They serve as travel corridors linking rivers, wetlands, and larger blocks of forestlands. Trees provide shade, which cools the water flowing beneath them. This improves the aquatic habitat by lowering stream temperatures, increasing the amount of oxygen in the water, and reducing evaporation from the water. Overhanging roots and branches also provide good fish habitat and organic inputs into stream systems from falling leaves and small branches. This organic matter serves as the basic food source for fish and other aquatic life. Larger organic debris, such as large branches, root wads, and fallen trees also provide cover and serve as feeding and resting places for macroinvertebrates, amphibians, and reptiles.

Riparian buffers are aesthetically pleasing and provide many recreational opportunities such as hunting, fishing, bird watching, and wildlife photography. Well-managed riparian buffers also can provide economic returns from quality timber for commercial harvesting.


Riparian Forest Buffers
Oklahoma Cooperative Extension Service
Water Quality Series

Introduction

Riparian areas are the lands directly adjacent to water as shown in Figure 1. Riparian areas include the trees and other plants growing next to a stream, river, pond, or lake. These areas may be a few feet near streams or a mile or more in floodplains. Both perennial and intermittent streams support riparian vegetation. Because riparian areas represent the interface between aquatic and upland ecosystems, the vegetation in the riparian area commonly has characteristics of both aquatic and upland habitats. Many of the plants in the riparian area require plenty of water and are adapted to shallow water table conditions.

Due to water availability and rich alluvial soils, riparian areas are very productive. Tree growth rate is highand the vegetation under the trees is usually lush andincludes a wide variety of shrubs, grasses, and wildflowers. In Oklahoma, bottomland hardwood forests aregood examples of riparian areas.

Riparian areas have many functions. Most notably,riparian areas can
1) store water and help reduce floods;
2) stabilize stream banks and improve water quality by trapping sediment and nutrients;
3) shade streams and help maintain temperature for fish habitat;
4) provideshelter and food for birds and other animals;
5) supportproductive forests which can be periodically harvested;
6) can be used as recreational sites; and
7) provide productive pasture lands for livestock.

Streamside forests are critical to the protection and en­hancement of water resources. It is important that a streamside forest’s capacity to provide the products and habitat is not reduced. Many of these areas are best managed as forests, rather than converted to other uses.

Benefits of Riparian Areas

Riparian areas provide different values to society. Allriparian areas do not develop the same way and can havedifferent sizes and shapes. Some are very productive andothers are not. The following is a brief discussion of some basic values that riparian areas provide.

Flood Control

Riparian areas associated with river and stream flood-plains act as water storage areas that can significantlyreduce the height of floods downstream. In addition, theabundant vegetation found in riparian areas can helpreduce flood velocities. This is why maintaining theseareas as forest lessens flood damage.

Healthy riparian areas act like a sponge. When floodwaters are slowed, these areas allow some of the excesswater to enter underground water storage areas. Slowrelease of stored water from riparian zones helps main­tain streamflow between storms. Establishing properdrainage, filling river and stream floodplains, and modifying water courses decreases flood storage, thereby increasing both the number and intensity of floods that cause severe erosion and property damage. Many of thesame attributes that enable riparian areas to reduce flooding also reduce stream bank erosion. Vegetation slowswave action and roots of trees help bind and stabilize thesoil.

Fact Sheet #1: Functions of Riparian Areas for Flood Control
Russell Cohen, Rivers Advocate
Riverways Program
Massachusetts Department of Fish and Game.

How do riparian areas provide flood control?

Naturally vegetated riparian areas (uplands as well as floodplains) serve a number of beneficial functions for flood control. An undeveloped, vegetated floodplain reduces the force, height and volume of floodwaters by allowing them to spread out horizontally and relatively harmlessly across the floodplain. Water that floods into vegetated floodplains reenters the main channel slowly, enabling it to be soaked up by the "sponge" of floodplain wetland soils and streamside forest leaf litter. Living, decaying and dead vegetation on riparian lands that falls or extends into the water provides numerous barriers against moving water, which slows it down so water is not delivered downstream as quickly. Such vegetation also intercepts and detains runoff from adjacent upland areas that would otherwise flow directly into rivers and exacerbate flooding conditions downstream. The root systems of streamside forest and emergent aquatic vegetation keep pores of the soil open so that two to three times more water can infiltrate the soil compared to lands used for cultivation or grazing.

In addition, trees, shrubs and herbaceous plants in riparian areas use large amounts of water in transpiration, which, in effect, transfers floodwaters to the atmosphere. Several thousand gallons per acre of water are used by plants each day, thereby drying the soil and making more room in the "soil sponge" for floodwater. The combined effect of all of these functions is a significant reduction in peak flows and flooding downstream. Naturally vegetated riparian forests thus help prevent thousands of dollars in property damage and obviate the need for human-made flood control measures and structures.

What alterations to riparian areas impair their ability to provide flood control?

Removing streamside forests from riparian areas impairs their ability to provide flood control in several ways. Floodwater detention is substantially reduced by removing the natural barriers of live, decaying and dead vegetation from the forest floor. Removing streamside forests will also result in an increase in soil compaction and reduction in soil porosity. All of these impacts combine to cause a significant decrease in infiltration and a subsequent increase in the speed and amount of flood runoff. Furthermore, floodwater reduction through transpiration is likely to be reduced, as grass transpires much less water than forest vegetation. Last but not least, excessive sedimentation resulting from the removal of vegetative cover from riparian areas reduces flood storage, as eroded sediments settle out of the current and fill channels and deeper spots on the river so they can no longer convey or hold as much water. This reduction of storage capacity increases peak discharges and the likelihood of flood damage.

...Increased watershed imperviousness typically amplifies normal streamflow patterns. Not only does it result in "higher highs" as stormwater, with little or no opportunity to infiltrate the soil, is directly discharged into rivers and streams, exacerbating downstream flooding, impervious surfaces result in "lower lows" when, during periods of low precipitation, the lack of previous infiltration into groundwater reduces baseflow and causes many urbanized streams to stagnate, putrefy and even dry up completely. Stormwater management systems for impervious surfaces typically only mitigate (lessen the severity of) but do not eliminate this problem.

...Structural attempts at flood control, such as confining watercourses into narrowly corseted channels and levees, have the effect of raising both the velocity and the height of any subsequent flood flow and make it all the more frightening and destructive when a river breaks through defenses, as has recently and repeatedly happened in the Midwest. [The effect can be like that of a bursting dam.] Channelization of meandering streams cuts the water storage capacity of those streams and causes water to flow more rapidly downstream, exacerbating flooding and storm damage downstream. An increase in water velocity also increases a river's erosive power, placing bridge supports, embankments and other vulnerable areas at greater risk of being undermined. Even if channelized river segments and, on a smaller scale, drainage pipes for a low-lying development, have the effect of conveying flood flows quickly away from that particular location, they will nevertheless increase peak flows downstream. In other words, the cumulative impact of altering riparian areas for the purpose of accelerating the rate at which floodwaters move downstream will exacerbate the severity, duration and frequency of flooding events downstream: the flooding problem is not controlled; it is merely relocated from one spot to another.

Why are vegetated riparian areas along smaller streams as significant for flood control as along the larger rivers?

A large proportion of the water in the state's rivers is contributed by the smaller tributaries. If riparian areas along these brooks and streams are altered in a manner that impairs their flood control function, the cumulative impact of streams discharging flood flows into rivers at a greater volume and velocity will result in worsening flooding in mainstem river communities, even if mainstem floodplains are safeguarded against further development. Furthermore, the same development is likely to have a relatively greater negative impact on flooding conditions in an adjacent small stream than the same project would have along a larger river (e.g., the runoff from one large parking lot can itself be enough to overwhelm a small stream channel).

What are some additional benefits of keeping floodplains naturally vegetated?

Maintaining or reestablishing vegetation in riparian areas increases the water-holding capacity of soil, which helps to recharge groundwater supplies. The slowing and dispersal of runoff and floodwater by floodplain vegetation allows additional time for this water to infiltrate and recharge groundwater aquifers. Floodplain soils and vegetation can also help to purify the water as it filters down to the aquifer. Once there, the groundwater can reemerge and seep back into surface water during the drier months and help reduce the frequency and duration of low streamflows and the higher temperatures, higher pollutant concentrations and other adverse conditions to riverine ecosystems brought on by periods of low streamflow. Last but not least, naturally vegetated floodplains function as important fisheries and wildlife habitat (see respective fact sheets for details).