Keeping the Connection
Usually, the only time floodplains—the flat or low-lying areas next to streams—come to our attention is during floods. When rising waters are not threatening property or lives, it's hard to remember floodplains exist, much less how important they are to the health of our streams.
As they meander through valleys, streams naturally create their own floodplains. No heavy equipment or detailed project plans are needed!
These low-lying areas are called floodplains because they naturally flood when water levels top the stream's banks. This function of collecting excess water is important. It reduces the amount of water that surges downstream.
Flowing water contains a lot of energy. The faster it flows, the more erosive and damaging it can be. Floodplains allow water to spread out and slow down, reducing its potentially destructive force.
Although much of the water that disperses into floodplains eventually returns to the river, some of it soaks into the ground. The amount of infiltration that takes place depends on soil types, land use and vegetation. Water that soaks into the ground doesn't race downstream, where it can cause damage. As a bonus, floodplain infiltration helps recharge our groundwater supplies.
As the water that enters a floodplain slows, the sediment it carries drops out. Each flood brings more sediment, enriching and deepening the soil.
Forests in floodplains are used by wildlife more than any other habitat type. These areas are home to mammals, reptiles, amphibians and insects. Many fish species also use flooded areas for spawning, nursery sites, feeding and refuge.
Vegetation, leaves, twigs and other organic material washed from the floodplain add nutrients to the stream system. These materials form the base of a stream's food chain, providing forage for insects and crustaceans. Just as a stream nourishes its floodplain, a floodplain nourishes its stream.
Signs of Disconnect
Given the importance of floodplains to streams, and the natural interaction of the two, it seems ill-advised to separate them. Yet we've found many ways to disconnect streams from their floodplains.
For example, many of our streams are lined with levees. These embankments contain the flow of streams and keep the water out of the floodplain. Levees often protect urban, industrial or agricultural areas, but at a price. Depending on the height of the levees, their distance from the stream and whether they constrict both sides of the stream, they interfere in some way with the natural function of the floodplains.
Simply clearing vegetation from the floodplain reduces its ability to slow floodwaters and filter them. Not only will flood currents be faster and more destructive, but without vegetative roots to help hold them in place streamside soils also will be more vulnerable to erosion.
We've also dammed rivers. Both flood control and hydroelectric dams change stream flows dramatically. Many of these rivers no longer have a natural stream flow. Instead, their levels rise and fall based on flow release schedules from upstream dams. The benefits of periodically inundating these floodplains has been reduced or completely eliminated.
Smaller dams built to create lakes or ponds also change water flow, although their effects aren't as dramatic as big dams. The impact of numerous small dams can be seen during drought years when they hold runoff, which is then lost to evaporation.
Often overlooked but hugely important are the high flow channels in floodplains. These secondary channels contain water only when stream levels are extremely high. They are necessary for storing excess flow and scattering energy.
High flow channels only function sporadically so people sometimes believe that it doesn't hurt to fill or block them. The result is that all the water and energy of even moderate floods remains in the main channel where it can scour and erode stream banks.
What Can We Do?
The floodplains of most of our developed stream systems have already been altered, but that doesn't mean we should give up on them. Just because past disconnections have taken place doesn't mean we can't work to improve the connections. They just require some special effort.
For example, we can “notch” levees or set them back from the stream to allow for periodic inundation of the floodplain. The benefits here will be greater where floodplain areas have not been highly altered.
Many riverside communities have learned to use their floodplains for parks, ball fields, agriculture and other facilities that can withstand some flooding. During future floods, their recovery effort and expenses will be far less than if the floodplains were filled with homes or businesses.
One benefit of these kinds of low-impact uses is that vegetation usually remains on riparian corridors, the areas immediately adjacent to the stream banks. In the absence of a healthy, well-vegetated, functional floodplain, riparian corridors become especially important in controlling water temperatures, filtering and storing excess nutrients and sediment, providing wildlife habitat and stabilizing soils.
We also need to avoid blocking or filling high-flow channels and to remove natural or constructed blockages already in place.
It may be feasible to remove smaller dams or dams that are no longer functioning. Varying flow levels on larger dams would allow downstream areas to experience natural, seasonal fluctuations in water levels. We can also modify dams so they don't hinder fish movement.
The Ultimate Goal
Streams eventually flood. Squeezing a river and depriving it of its natural floodplains is like thumbing the end of a water hose. It dramatically increases the force of the water, making even a minor flow powerful enough to cause damage.
Floodplains naturally regulate rivers and buffer floods. When allowed to remain connected to the stream, floodplains benefit our natural stream habitats, as well as the budgets and infrastructure of our streamside communities.
It often takes a flood to remind people of the value of floodplains to a stream or river, but we don't have to wait until disaster strikes to take action. Communities that keep streams and floodplains connected suffer less damage when the water rises, and their citizens have a wonderful resource to enjoy and appreciate throughout the year.