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An overlooked carbon sink? The influence of rivers and floodplains on the carbon cycle

Written by Katherine Lininger, a 2016-2017 Sustainability Leadership Fellow and Ph.D. Candidate for the Department of Geosciences.

Carbon is all around us— it is an essential component of air, and it forms the foundation for all living things. Carbon is taken from the atmosphere by plants through photosynthesis, incorporated into soils by organisms, and released to the atmosphere during decomposition of organic matter. The carbon dioxide that we emit by burning fossil fuels warms our planet and disrupts our climate. Having a grasp of the global carbon cycle, or how carbon moves between the land, ocean, and the atmosphere, is important for understanding human-induced climate change. As we emit more carbon dioxide into the atmosphere, we can’t accurately predict future warming unless we know where carbon is, how it moves, how likely it is to go into the atmosphere, and where it is stored on the earth. In other words, we need a better understanding of the carbon cycle in order to understand how humans are modifying the climate by releasing carbon into the air.

Rivers play a role in the carbon cycle because they erode the landscape and carry soil and carbon from the land to the ocean. As carbon moves in and along rivers, it can decompose and be released into the atmosphere as carbon dioxide. Eventually, rivers can deliver carbon and soil to the ocean. Along it’s path from the land to the ocean, however, carbon can also be deposited in floodplains and stored for up to thousands of years. Unfortunately, the ability of river channels and floodplains, together known as river corridors, to store carbon has been reduced due to human modifications of our river systems.

The role of river corridors in the carbon cycle

The amount of carbon stored in soil globally is about twice the amount that is in the atmosphere. Needless to say, soil carbon is a large and important stock within the global carbon cycle. As described by SoGES sustainability fellow Derek Schook, rivers erode and transport soil from the land, moving it around and depositing it elsewhere. Rivers also transport and deposit dead trees (known as large wood), and about half of the mass of large wood is carbon. Carbon that enters a river has three potential fates. First is that bugs and microbes will eat the carbon as an energy source, eventually releasing it to the atmosphere. Second, some of the carbon that enters rivers will make it out to the ocean and be buried in ocean sediments. A third fate of carbon in river corridors is burial in floodplains, which results in carbon being stored for long periods of time.

As a PhD candidate, I am determining how much carbon is stored in floodplain soils in the Yukon River Basin in central Alaska. My research is part of a larger research initiative looking looking at carbon in many different river corridors. We are trying to figure out what physical characteristics of rivers result in carbon storage in river corridors. How do rivers erode and deposit carbon in their floodplains, and what parts of the floodplain store the most carbon? How long does carbon stored in floodplains stay there before being re-eroded by the river? How important is carbon storage in floodplains and rivers compared to carbon storage in other parts of the landscape? These are the questions that will help us better understand the role of river corridors in the global carbon cycle. Little is known today, and we are only beginning to answer these questions.

Human activities have likely reduced carbon storage in river corridors

A striking example of humans reducing floodplain carbon storage can be found in a study of the lower Mississippi River valley. The scientists estimated that the amount of carbon stored in floodplain soils and floodplain forests is only about 2% of what was historically present before agricultural development and river modification.  Worldwide, we have modified river corridors extensively by damming and extracting flow from rivers, modifying river banks, removing large pieces of wood and beavers from rivers, building levees and disconnecting the river channel from its floodplain, and clearing native vegetation to allow for agriculture and urban development.

Floodplain soils are good places to grow crops—they are nutrient rich and full of carbon because the river has delivered sediment and nutrients over long periods of time. Without human modification, wet and messy floodplains have the potential to store a lot of carbon. Decomposition, which releases carbon into the air, is slow when soil is submerged by water, and physically complex floodplains with lots of oxbow lakes, side channels, and space for sediment provide good trapping areas for carbon. Pieces of large wood, either by themselves or in large accumulations, trap sediment and help to create more complex floodplains. Similarly, beavers create extensive wetlands, storing sediment, nutrients, and water.

But, if we simplify river channels and their floodplains, river corridors likely store less carbon. Damming rivers and extracting flow can reduce flooding, cutting off delivery of wood, organic material, and sediment to the floodplain. Building levees also cuts off the connection between the river and floodplain, and removing large wood reduces the physical complexity of rivers. We have drastically reduced historic beaver populations through beaver trapping, removing an animal that actively promotes storing carbon on the landscape by creating beaver ponds and wetlands. Replacing native floodplain vegetation with crops and urban development reduces the amount of carbon stored in plants in the floodplain. The carbon in river corridors doesn’t just disappear when humans modify floodplains and channels—it can be added to the atmosphere, joining the other carbon molecules emitted by burning of fossil fuels. So, when thinking about managing atmospheric carbon and greenhouse gases, we shouldn’t forget about all of the potential storage areas for carbon on the land, including river corridors.

Can we manage rivers and floodplains to enhance carbon storage?

Although river corridor restoration efforts have yet to explicitly state that enhanced carbon storage is a main goal, there are examples of floodplain restoration that are increasing carbon storage. For example, The Nature Conservancy is restoring floodplains along the Illinois River at Spunky Bottoms and Emiquon. In these areas, the river and its floodplain are being reconnected, and floodplain wetlands are being re-established. Internationally, the Murray-Darling Basin Authority in Australia has been restoring the Barmah Floodplain forest, allowing for water releases from dams to flood the floodplain. There are many ecological benefits to restoring the connections between rivers and floodplains and promoting messy, physically complex river corridors. But, an overlooked and added benefit of floodplain restoration could be increased carbon storage on the landscape. In order to better understand the impact that river corridors have on the carbon cycle, we need more research on carbon in river corridors.

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