Saturday, February 11, 2023

How Fallen Leaves Can Reduce Flooding

A kindred spirit in Durham, NC, naturalist Riverdave Owen, took the time last fall to count the fallen leaves freshly fallen on one square yard of his land near the Eno River. 

Posting this photo on facebook, he said the count totaled 1600 leaves. "After extrapolating that count, I estimate this year's autumn leaf drop for this entire half acre property to be approximately THREE MILLION LEAVES! Instead of raking, blowing and hauling them away, they are left as food for a myriad of tiny intermediaries who then return the foliage's organic nutrients back to the trees from whence they came .."

As Riverdave points out, the habitat created by leaves certainly is important. Just think of the spectacular increase in surface area they provide when they fall on the ground. Each leaf has surface area on front and back, and as they accumulate they create myriad tiny spaces inbetween. 

An additional benefit of leaves that tends to go unmentioned and unexplored is the impact of leaves on stormwater runoff. If I had the patience to collect data, I would take that square yard of leaves, weigh it when the leaves are dry, then soak the leaves and weigh them when wet. The difference in weight would be the amount of rainwater sequestered by the leaves themselves during a storm. Multiply that by all the square yards covered by leaves in a community, and you'd find out how much water is absorbed by leaves rather than contributing to downstream flooding. 

But leaves do more than absorb rainwater. They also help make the ground beneath them more absorbent. Exposed soil can harden and dry out, making it behave more like impervious concrete when the rains come. If instead the ground is covered by leaves, the leaves' slow decomposition softens the soil with organic matter and promotes soil life whose activity in turn opens up and maintains channels for water to seep into. 


  1. I often wonder why my suburban neighbors rake their leaves and haul them out to the curb for the township trucks to take to the landfill.

    There's much about this that's wrong headed, except of course that the effort provides exercise that many of us need. But there are less destructive ways to exercise.

    Destructive? Yes indeed. When leaves are buried in a landfill, anaerobic bacteria act on the organic matter and produce methane gas, a potent greenhouse gas that's about 28 to 84 more potent than carbon dioxide, depending on the time scales considered. If the leaves are instead mulched with a mulching blade on a lawnmower, the earth digests the leaves in the presence of oxygen, producing carbon dioxide and food for whatever plants are in the soil.

    I'm amused when I see neighbors working hard to pile their leaves on curbside, when all I do is wait until most all the leaves have done their yearly transition from trees to earth, then do a final mulching mow for the year. The funny part is that my lawn looks very healthy and in the Winter, my lawn is as cleared as leaves as anyone else's. One reason for that is because the wind moves all the leaves in the neighborhood wily nily without regard for who or who not raked their leaves. So why do they do that leaf collecting ritual? Beats me. I'm sure it makes them feel like good citizens.

    Consider also the energy it takes to truck those leaves to the landfill. What's the point of it, especially since you rob your soil of the nutrients all leaves contain? But then again, it provides work for township workers.

    Such are some of the mysteries of suburbia in America.


    1. Good description, and I like you're strategy. As far as I know, the leaves don't go to the landfill, but instead go out to the composting site on Princeton Pike. That intense decomposition makes nice compost, but also speeds the return of the carbon in the leaves back to carbon dioxide, whereas in the yard the leaves would sequester carbon longer. From what I hear, there are a lot of public works tasks left undone while crews are chasing leaf piles on the streets, which is why the town says it's cutting back on pickups.

  2. True -- Autumn should be the season for the piling on of work (in harvesting!), rather than the working on of piles (of leaves).