“No one has cleaned these rivers”. That’s what a landscape architect once told me when he saw fallen trees in the rivers in a national park. It bothered him that “no one had taken them out yet”. But…why would we remove them?
Depending on the river, its uses, country, and even season, trees that at some point fall into rivers are usually removed to “aid” in navigation. According to Roni et al. (2014), before the 1980s they were also thought to impede fish migration. This, we know now for sure, is not true! Also, not all rivers are used for (heavy) navigation.
Plant material, in particular large woody debris (LWD), plays many important roles in rivers. They supply allochthonous nutrients (those which originate outside the river) and aid in the retention of nutrients. The new “barrier” alters the flow of water. This changes the dynamics of erosion and sediment accumulation on the river bottom and banks. It creates new habitat for aquatic invertebrates, fish, amphibians, and even birds, as well as sites for fish to hide from the latter and from other aquatic mammals.
From nutrient cycling, hydraulics, morphology, and even life in the river, allowing the logs to stay in the river is an excellent idea. “Adding” them is even a common measure in river restoration.
So now you know, the next time you see logs in a river, instead of thinking that they “look ugly”, get closer and see who lives there and appreciate their important functionality.
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Ref:
Roni, P. et al. 2014. Wood Placement in River Restoration: Fact, Fiction, and Future Direction. Canadian Journal of Fisheries and Aquatic Sciences.