While Trout Unlimited’s river and floodplain restoration is often aimed at trout and salmon species, the benefits of our work extend far beyond fish habitat.
When a stream channel is reconnected to its floodplain, key complexity and ecological processes are restored. The riparian habitat becomes more diverse, composed of a mosaic of side-channels, wetlands, willow thickets, aspen stands and beaver ponds. Such complexity helps slow floodwater when the river jumps its banks, protecting downstream communities.
Slower water means more infiltration into soils, supporting riparian vegetation, replenishing ground water, releasing cleaner water downstream and providing drought-resilience for water users. Intact floodplains also provide important habitat for birds, amphibians and other wildlife. And during wildfires, saturated floodplains are less likely to burn.
Alongside these positive changes to stream health and the surrounding landscape, less visible processes may also be taking place. This summer, TU’s National Science Team is investigating one such benefit: carbon sequestration.
Floodplains store carbon
Carbon sequestration within intact floodplains occurs as the combined result of the benefits described above—slow water, trapped sediment and robust vegetation. Research from natural, undisturbed floodplains across North America indicates that while floodplains make up only a small portion of Earth’s surface, these critical river habitats store a disproportionate amount of carbon.
Floodplains store carbon in three major pools: living biomass (shrubs, trees, grasses); dead biomass (large wood); and soil carbon. The latter, soil carbon, is the largest, longest-lived carbon pool, potentially storing carbon for thousands of years.
Not only are intact floodplains big, robust carbon reservoirs, but they capture carbon rapidly. Recent research indicates that wetlands may sequester carbon at 30-50 times the rate as forests (Tangen and Bansal, 2020).
Therefore, river restoration projects that reconnect a stream channel to its historic floodplain and spread water across formerly dewatered areas may also restart long-dormant carbon pumps, drawing down carbon dioxide from the atmosphere in the process.
The science, however, remains uncertain for now as to how much carbon might actually be sequestered by river restoration, how fast it occurs and how long it is stored.
New stream restoration research
With the support of the Paul G. Allen Family Foundation, Trout Unlimited’s National Science team is taking these critical questions head on.
The team, led by Jordan Fields, TU’s aquatic resiliency scientist, has selected a suite of stream restoration sites across Oregon and Alaska where they will measure both standing carbon stocks and sequestration rates.
A carbon stock refers to the amount of carbon stored in a floodplain at one moment in time. To assess the carbon stock at a restoration site, the science team is collecting soil cores to measure soil carbon and using drones paired with ground-based surveys to measure carbon stored in living biomass and dead biomass.
Adding up the carbon in these three pools—soil, living biomass, and large woody debris—allows for estimation of carbon in tons per acre.
The team is also measuring the age of soil layers to determine how quickly carbon is being captured from the atmosphere and stored in soil. To determine the effectiveness of restoration with respect to carbon, the science team is employing these methods at a variety of restored, degraded and intact sites (natural, healthy floodplains) to compare the results.
And fieldwork is just the start of the process.
Upon returning to the lab, the soil samples need to be dried, weighed, sieved to remove gravel, crushed and milled to create a homogenous sample, and then sent off for total carbon analysis and age dating. When the lab work is complete, all the data gathered from hundreds of samples needs to be analyzed to understand the results.
Stay tuned
This summer the science team is hard at work visiting restoration projects, gathering samples, mapping sites and processing samples in the lab. Afterwards, they’ll be running the numbers and, hopefully, will have exciting findings to report later in the year.
This work provides a crucial, additional scientific perspective on stream restoration. As TU’s restoration projects rebuild habitat for fish and wildlife and help restore the ability of intact floodplains to store the cold, clean water fish and communities depend on, we’ll also gain a better understanding of how reconnected floodplains interact with carbon in the atmosphere and how these critical riparian ecosystems act as important sources of natural carbon storage.