What’s a stream to look like?

 For quite some time in many people’s eyes a perfect stream was a clean stream. That meant no tangled roots and woody debris in the water to obstruct the natural flow.

 To satisfy this expectation, we pulled trees out of rivers and brooks after they fell in or got washed in by storms. Commonly, the best photographs on our nature calendars showed no mess.

It wasn’t just aesthetic preference that led us to remove tree trunks and branches from waterways. Downed trees can pose navigation hazards, and safety hazards, too. Strong currents can pin swimmers and kayakers against or beneath fallen trees, and also send heavy wood crashing into bridges.

There’s been rethinking about woody debris in streams thanks to advances in river science, however.

Here’s a good explanation by Emily Howe, a writer in Cool Green Science, the conservation science blog of The Nature Conservancy:

 “In floodplain valleys and estuarine deltas, large log jams are island builders and river dividers. Log jams at the head of gravel bars eventually become large, forested islands.

“These islands force the river to meander across floodplain valleys, cutting through riparian forest in a braid of channels that reinforces the cycle of erosion, treefall, and island formation. The cycle ensures a complex mosaic of gravel bars, islands, floodplains, swift river chutes, quiet pools, and backwater streams — a kaleidoscope of habitats that offer salmon refuge from predators, feeding and spawning grounds, and safe routes to the sea.

“The entire process hinges on constant movement and constant change, creating an ecological portfolio that rivals the best the stock market has to offer.”

Here’s another comment that focuses on the biological benefits that come of logs in water, this one from Fishbio, a California-based fisheries and environmental consulting company:

“Logs and logjams benefit fish by interrupting the downstream flow of water. Nutrients such as carbon and nitrogen, which are normally quickly flushed downstream, get trapped in the pool areas upstream of logjams, stimulating the growth of algae and plants at the base of the food web.

“This in turn produces more macroinvertebrates that serve as fish prey. In and around logs and jams, fish benefit from pockets of slower moving water that allow them to conserve energy while watching for drifting prey in the main current. Logs also provide overhead cover for adults and juveniles from hungry birds, and the deep pools that form behind large wood can serve as refuge spots during harsh summers.

“Logjams force water to flow under and over them, which increases the amount of water that seeps into the ground or spills out onto floodplains. Logs themselves can also foster the sprouting of young trees, supporting the next generation of riparian forest.”

 These discoveries have led to new practices around streams, brooks and rivers in which we’re not merely leaving downed trees in the water but actively putting in large wood.

In fact, that’s the term of art for the tree trunks with tangled root structures that are increasingly being put into waterways: “Large Wood.”  A good history of the term and practice can be found in the Journal of the American Water Resources Association under the title “Management of large wood in streams: An overview and proposed framework for hazard evaluation.”

The article, written in 2016, describes how engineers are working to accommodate the change of thinking by doing such things as designing bridges with wider spans so as to allow floating logs to pass beneath them without getting stuck, and also, in the opposite case, anchoring tree trunks in streams to keep them from being swept downstream.

Such steps leave streams and rivers looking different; by traditional standards, they leave them looking a mess. In many respects, such is Nature. In many respects, such is life.


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