Most rivers in the developed world are mere artifacts of what they once were – wild dynamic beasts that wiggled across the landscape recycling old land and creating new surfaces. With human development came the desire to manage water movement, and now most rivers flow through static channels held in place by levees and controlled by dams. But recently some regions are making efforts to give rivers a bit more freedom and restore their associated floodplains.
Rivers and Their Floodplains …What’s a Floodplain?
Rivers meander and migrate across the landscape, like a snake, changing course through erosional and depositional processes that are easily seen with the aid of time lapse imagery (Fig. 1). Because of this dynamic nature, the area immediately around rivers – known as the floodplain – is exceptionally productive and rich in nutrients (Fig. 2). This is a direct result of the river reworking the floodplain soils via meander migration, along with annual flood events that deposit new sediment across the floodplain. For example, the Mississippi River floodplain is the most fertile soil on the North American continent and produces 92% of U.S. agricultural exports (USNPS 2017).
However, at this point in time, most rivers can no longer migrate and some do not even meander anymore. A series of levees (constructed by humans) confine most rivers to a fixed channel; multiple dams occur on most rivers controlling the flow levels (Fig. 3). These modifications prevent rivers from moving and flooding onto their floodplains. This is of course to protect the developed and agricultural land along floodplains. Flooding, after all, remains the most damaging natural disaster worldwide with 100 million people affected annually (Opperman et al, 2017). Yet without flooding, and without river migration, we have also lost all the processes that made floodplains such valuable land in the first place. Additionally, if floodplain land is so obviously beneficial for agriculture, can you imagine how lush and biologically diverse the land was before humans converted it? The Central Valley of California, a historic floodplain, once had four million acres of wetlands that supported millions of migratory birds (Frayer et al. 1989). Today, we have lost 90% of that bird habitat. By strictly controlling rivers, not only have we lost the architects of healthy soils, but we lost much of the valuable floodplain habitat that rivers supported.
However, things are starting to change. In some parts of the world, we are restoring rivers to revive the processes that made floodplains so valuable to begin with. This often means widening the river levees and reconnecting rivers to floodplains to promote meander migration. One of the biggest challenges with this effort is being able to do it in such a way that still protects areas of the developed floodplain. Clearly we cannot simply cut holes in levees and expect good things to happen, especially if we have cities on the other side! Thus, we must limit our restored areas. So if we cannot completely restore river-floodplain connections, will our limited restoration efforts even be worthwhile?
A Recent Restoration Example
In a recent study, researchers attempted to answer this question about a restored river in Spain (Martinez-Fernandez et al. 2017). They compared three river sections: An unregulated, “natural” river; a restored river; and a control river that was regulated and modified. They wanted to see if the restored river was an improvement from the modified river, and if it displayed some of the positive characteristics of the natural river. To do this, they compared aerial imagery of each river section in 2011 and again in 2014. Using spatial mapping software, the researchers mapped the area around the river and categorized it as either active channel (where the water flows), gravel bars (banks, small islands or shallow areas in the river), or vegetation. Then they looked at how each of those areas changed between the two different time periods.
They found that the restored river section had a much more active channel area compared to the other two river types. The wider space for the restored river allowed it to move more, and it did this by cutting into its banks and recycling sediment. This allowed the river flow to carry and deposit sediment creating a more diverse landscape consisting of gravel bars, vegetated areas, and a more dynamic channel. However, as the authors stated, these positive effects were limited and the restored area is still much smaller than the original floodplain prior to regulation.
Another interesting result was the restored channel had more new vegetation compared to the modified river. Because the restored channel has a chance to carry and deposit sediment, it is actively creating new gravel bars and habitat for various species (e.g., Fig. 4), whereas the modified river section is more confined with less renewed land for plant colonization.
So What is the Take-Home Message?
Floodplains make up less than 2% of Earth’s terrestrial surface, but they provide 25% of all non-marine ecosystem service benefits. Most importantly, they reduce flood risk, but other benefits include curbing sediment and nutrient pollution, carbon sequestration, groundwater recharge, fisheries, recreation, improved water quality, and biodiversity (Costanza et al. 1997). All of these benefits rely on a connection between the river and its floodplain. This study demonstrated the positive effects of one of the most ambitious floodplain restoration projects in the world to-date: Increased river migration and recycling of sediment, renewed floodplain surfaces, better vegetation structure, and potential for greater species diversity. Although this project only reconnected a small fraction of the river’s former floodplain, these results are encouraging evidence that reconnecting floodplains to their channels can provide measurable environmental benefits.
Costanza, R., R. d’Arge, R. de Groot, S. Farber, M. Grasso, B. Hannon, K. Limburg et al. 1997. The value of the world’s ecosystem services and natural capital. Nature 387 (6630):253-260.
Frayer, W.E., D.D. Peters, and H.R. Pywell. 1989. Wetlands of the California Central Valley: Status and trends 1939 to mid-1980s. U.S. Fish and Wildlife Service.
Martinez-Fernandez, V., E. Gonzalez, J.C. Lopez-Almansa, S.M. Gonzalez, D.C. de Jalon. 2017. Dismantling artificial levees and channel revetments promotes channel widening and regeneration of riparian vegetation over long river segments. Ecological Engineering 108:132-142.
Opperman, J.J. et al. 2017. Floodplains: Processes and Management for Ecosystem Services. University of California Press Books. Oakland, CA.
U.S. National Park Service. 2017. https://www.nps.gov/miss/riverfacts.htm. Accessed 1/12/2018.