Written By George Johnson.

Posted on October 2nd, 2017.

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You might look at a leaf-covered stream and think it looks dirty. But all that leaf litter is good for water quality, both for creatures in the water and those of us who depend on it to drink.

A couple autumns back, I came across this stream and noticed that it had almost completely changed overnight. The leaf litter from the surrounding trees had all washed into the water. The stream that the day before looked like it could be the logo for a bottled water company had transformed into a sad-looking trickle jam-packed with leaves.


Almost every small brook in the Northeast goes through this transformation during autumn. The trees along the bank drop their leaves as they prepare for winter. Those leaves have to end up somewhere once they fall, and that somewhere is often in the stream.

You might look at a stream like this and think that it looks dirty. You might even think the water quality has dropped owing to all this new “litter.” But as it turns out, all those leaves are good for water quality, both for the creatures living in the water and those of us downstream who depend on it to drink.

How is that possible? During the year little sunlight reaches the water in small springs and creeks. The trees growing along the banks are generally large enough to cover the entire stream. Because of that, algae and other aquatic plants that would normally form the base of the stream’s food chain have a hard time living. What do all the critters in the stream eat? You guessed it: those leaves. Insects in the water like caddisfly larva tear apart, filter, and eat all that plant material. In turn, larger creatures like fish eat those insects. Far from trash, autumn leaves are like a smorgasbord falling down on streams.

But how do those leaves make water quality cleaner for the rest of us? Thank all those insects that eat the leaves. As they eat, they also break down many common water pollutants like nitrates and ammonia into harmless elements. When that water flows downstream and eventually winds up in reservoirs, it’s cleaner and needs less treatment before it reaches our faucets.

So raise a glass to that leaf-filled stream. It may look ugly, but it’s really one big, nature-powered water filter.


Don’t have trees along your streams? Planting some is one of the best things you can do to improve wildlife habitat and water quality on your land. Check out these MyWoodlot activities on how to plant and maintain streamside trees.

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