Floodplain Restoration

Explore the problems and solutions associated with floodplain by clicking through the slides using the slide-navigation buttons () below.

The Problem

Restoration of a stream can never be completely successful unless the floodplain, riparian, and wetland areas associated with it are also restored. That's because the functions of the floodplain are intricately connected to the functions of the stream. The problems can stem from past agricultural practices, which often substantially altered floodplains—everything from their hydrology to their microtopography to the vegetation growing on them. Agricultural and other land uses often increase the sediment load entering the river, alter natural disturbance regimes like flooding and fire, introduce weeds, simplify topography and vegetation, and destabilize stream banks. The actions associated with stream restoration itself also require floodplain restoration measures.

The Solutions

The problem of floodplains not functioning properly has a number of solutions, depending on the situation and the specific alterations that have occurred. Remedies include halting or controling erosion, restoring natural floodplain and wetland topography, planting native species either by seeding or installing containerized plants, salvaging and transplanting trees and mature shrubs from the site, and revegetating and stabilizeing banks using bioengineering techniques. Click below to learn more about the solutions.

Many restoration projects require some level of erosion control until seeded or planted material is sufficiently established to maintain soil stability. This is especially true for streambanks, topsoil areas of the floodplain that frequently flood, and on slopes steeper than 3:1 (horizontal to vertical). Erosion control treatments include placement of logs, rocks, or other objects on slopes or in the floodplain to improve microtopography and reduce erosion, installing organic material layers (mulches), contour wattles (fascines), and manufactured erosion control blankets. Erosion control measures are often used with revegetation techniques.

Wattles

Contour wattles are used to intercept runoff and trap sediments. Wattles are cylindrical bundles of organic material such as plant fiber, straw, brush or live willow stakes, bundled together with twine or packed into sausage-shaped organic or synthetic webbing. They are typically 9 to 18 inches in diameter and 6 to 20 feet long. They are used on disturbed streambanks and steep slopes and will eventually biodegrade.

Blankets

Erosion control blankets protect the soil surface and are used for holding soil in place until vegetation establishes. They slow water at the soil surface and trap sediment and small debris, thereby building soil. They also intercept rainfall, slow precipitation runoff, and maintain surface moisture. On Jocko River projects, blankets made of organic fibers are preferred and limited to weed-free straw and coir (coconut fiber).

Logs

Floodplain and slope erosion control treatments may include placement of logs, rocks, or other objects on slopes or in the floodplain to im-prove microtopography and reduce erosion. Logs and rocks look natural and are effective barriers against ero-sion. These two photos show typical uses of logs for erosion control.

Microtopography

Enhancing microtopography means shaping the land surface—creating a rough or uneven surface—on the floodplain, which in turn creates all kinds of niches for vegetation. Even small elevational differences can substantially increase the diversity of the plant community. Excavating swales—larger depressions—with heavy machinery allows for the capture of seed, plant fragments that can root and start new plants, and debris during overbank or spring flows. These depressions also create islands of diversity and differences in soil moisture ranging from relatively dry to very wet conditions. Woodpiles trap wind and waterborne sediments, as well as plant fragments, and the area under and around woodpiles provide microsites to shelter young plants and this, in turn, can evolve into islands of diversity in the dominant plant community. The addition of larger diameter woody debris to the floodplain increases surface roughness on bare floodplain surfaces, which creates resistance to flowing water that in turn creates topographic diversity and microsites for plants.

Seeding

Direct seeding is the preferred method for establishing native grass and forb or herb cover over large areas, as these species typically germinate readily and provide cover quickly. Seeding may also be done to enhance the “seed bank” in the soil; using slower germinating species such as trees and shrubs to encourage the long-term recruitment of desirable native species. Successful seeding and establishment of native cover depends on many factors, including site and seedbed preparation, seed-mix design, application methods, post-seeding treatments and maintenance.

Container Plants

Appropriate native plant species are custom-grown in containers from locally collected seed or cuttings. Collecting them locally ensures the plants will be adapted to the site. Plants are planted in the kinds of places they would naturally grow—sites with the right soil moisture, soil type, sun or shade, etc. Once they have been planted out, workers make watering basins around each plant, install shade cards, browse protectors, mulch, and weed barriers. Then plants are carefully maintained until they become established, which might take one to three growing seasons. Maintenance activities include irrigating, controlling weeds, replacing dead plants, and maintaining browse protectors and shade cards.

Salvage

Plant salvage and transplant refers to excavating and relocating mature shrubs and trees or patches of sod for use in revegetation. Salvaging plants and native sod can be a relatively inexpensive and a quick method for obtaining large, native, site-adapted planting stock for rapid vegetative reestablishment and bank stabilization. Workers need to watch for weeds when identifying salvage stock and salvage during the right time of year, which is generally the dormant season.

Other

When grade control or other structures are installed in a stream channel, banks need to be engineered to hold the banks in place until they have enough vegetation and root stability to allow for natural channel migration. In other places, such as the banks along a riffle or run, planted vegetation is sufficient to stabilize the channel because there is less erosive force. But even in these places, the vegetation is often combined with a matrix, such as coconut fiber to provide short-term stability as roots establish.

All streambank bioengineering uses plant materials, and dormant willow cuttings are the most common material used because willows have the ability to root from the stem. When needed, benches are constructed at the bankfull level of the stream channel. These benches are then planted with sod, containerized plants, and/or cuttings, all of which speeds the reestablishment of vegetation and reduces bank erosion.

The Benefits

There are many benefits that come from restoring floodplains and riparian areas:

Healthy riprian areas provide shelter, water and food for wildlife—in fact, roughly eighty percent of Montana's wildlife use riparian areas for all or part of their lifecycle requirements.
Healthy riparian areas reduce the horsepower of streams and buffer the destructive impacts of floods by slowing water down through friction—a two-inch deep rootmat on a streambank resists erosion up to 20,000 times better than bare soils.
Healthy floodplains have a major influence on channel shape and size—well vegetated streams tend to be narrower and deeper.
Healthy riparian areas store water and release it when it's needed. For most streams, flow for late summer, fall, and winter months depends on groundwater storage in the floodplain.

Specific benefits of actions like erosion control, microtopography enhancement, plantings, seeding, and bioengineering include:

Reductions in the sediment load entering the river in disturbed areas.
More diverse habitat through increased surface roughness and diversity of the floodplain topography.
Promotion of more diverse and native plant communities and future seed banks and more stable soils.
The creation of environments along the river suitable for woody vegetation establishment.