PUBLISHED: 28 January 2014

Rewilding Our Rivers: The Ecological Argument For Healthy Soils And Their Role In Flood Mitigation by Lauren Eden

Welcome back to the discussion series Rewilding Our Rivers – a look at natural flood mitigation options for Alberta. This week we turn our attention to the importance of healthy soils and how the building of organic matter can play a significant role in flood mitigation.

Soil is made up of a complex web of life, where a single teaspoon of soil can contain billions of living organisms. Created from the functions of its many parts including bacteria, fungi, minerals, air and water, nurtured by sun making soil a living network from which all plant life sprouts.  

Healthy soil is considered one of the most complex ecosystems on the planet. Like all living organisms, soil requires nutrients (food), water, and air. Organic matter is essentially the soil’s food system created from decomposed organic material such as dead plants (carbon) or animal manure (nitrogen).  The decomposition of this matter into the soil system creates a topsoil layer called humus; a dark black living structure. Humus is essentially mature compost that has reached a state of stability, where it needs no further decomposing. Humus, a dense organic matter, significantly increases the soil system’s ability to retain nutrients and moisture.  Thus, healthy soil results in structure that is rich in organic matter enabling it to hold, filter, and purify water[1]

Ultimately it is the loss of this topsoil, the rich organic matter, which depletes a soil system’s ability to retain and store water. This loss of topsoil is due to erosion, mainly from deforestation practices (clear cutting) and the industrial agricultural sector (the cultivation of land using synthetic fertilizers and pesticides). These practices ultimately strip soils of organic matter leading to systems that are no longer able to retain or store water. Globally over the past 150 years, due to erosion inducing land-use practices, our soil structure has lost around 70 percent of its water holding capacity[2].

Numerous studies have demonstrated the importance soil structure plays in the role of flood mitigation. One study on a wetland’s ability to mitigate for flooding in the Upper Basin of the Mississippi River discovered that one unit of soil, rich in organic matter, can hold up to 31% of its ratio in water (0.31 inches of water per inch of soil); whereas soil that has been eroded holds only 4% of its ratio in water (0.04 inches of water per inch of soil)[3]. The study concluded if all soils throughout the Upper Basin of the Mississippi River were restored to systems rich in organic topsoil then flood flow volumes would be reduced by 45%[3]. Similar scientific findings were discovered in another study, conducted on the Redwood River Basin(a tributary of the Minnesota River), indicating that if 25% of soils in the Redwood watershed were restored to contain dense organic matter the result would be an average peak flood flow reduction of 35%[4]. Likewise, a study conducted on the Skunk River basin (a tributary of the Minnesota River) indicated that flow volume during peak flows of a 100-year flood would be reduced by 21% if land-use management practices, like increasing the concentration of organics in the soil, were applied fully. These findings speak to the practicality and importance soil restoration could play in the reduction of flood flow volumes in Alberta.  

Ultimately the rebuilding of our soils bleeds into land-use management and requires collaboration across government, industry, and watershed stewards. Rebuilding ecologies whether it is a wetland, a woodland, or soil structure differs from the mega infrastructure flood mitigation options. Natural solutions may be substantially cheaper to implement, but they also require patience as ecological succession returns. Sometimes, time is of the essence, and good solutions like the rebuilding of our soils are forgotten or forgone. It is important to not get lost in the quick fix and remember that rebuilding our soils creates a greater sense of ecological resiliency. This resiliency not only mitigates for future floods, but also helps to stabilize our climate; provides water retention in years of drought; and purifies our shared water resources. Valuing ecosystem goods and services like healthy soil structure is part of an ongoing paradigm shift. That is moving us out of “decision–making” silos and into an integrated land-use and watershed management future in which we begin to value the services these complex ecologies’ offer us.

Our soils hold and store our water supply. Do we as Albertans feel that it is important to restore the ecologies that retain our water? If so how should we proceed from here? How will we apply the rebuilding of organic matter to Alberta’s future flood mitigation plans?

Lauren Eden is a Researcher at Alberta WaterSMART

[1] Permaculture News. (2008). Soil Our Financial Institution retrieved from:

[2] Brady, N.C. (1990). The nature and properties of soils (10th ed.). New York, NY: MacMillian Publishing Company.

[3] Hey & Phillipi. (1995). Flood Reduction Through Wetland Restoration: Upper Mississippi River Basin as a Case History. Restoration Ecology: The Journal of the Society for Ecological Restoration, 3(1), 4-17. 

[4] Hunt, C.E. (1997). A Natural Storage Approach for Flood Damage Reduction and Environmental Enhancement. Onalaska, Wisconsin: U.S. Geological Survey.