Wednesday, November 16, 2011

Plants + Fluvial Processes = Fish

Plants are major factors shaping fluvial landscapes, acknowledged by the ever-popular Effects of large organic material on channel form and fluvial processes by Keller and Swanson (“fifth most cited paper in geomorphology” as of 2005).  Their examples include log jams widening channels as water is diverted around obstructions, and lodged debris producing mid-channel bars through deposition downstream; woody debris protects stream banks from erosion if lodged, but can also erode banks through scraping.  In Wood in Streams Anne Jefferson recently posted an interesting discussion with photos of an organic debris jam in the process of cutting off a meander.
Trout-fishing in the Laramie River -- 
“It sure felt like a really big one, you bet!”

But oddly ... there's no mention what must be the most critical role of woody debris in fluvial processes, at least in southeast Wyoming:  providing habitat for fish.

The Connecticut Department of Environmental Protection put together a fact sheet on large woody debris, in response to removal of LWD by well-intentioned volunteer groups while “cleaning up” river banks ... and degrading aquatic habitat in the process.
“Water flowing over and under LWD during high flow events can result in localized scour pockets or holes, providing excellent cover habitats for fish. LWD can also create velocity shelters for fish, especially behind large rootwads. Fish often rest within these velocity shelters, where water velocities are slower. In large streams and rivers, LWD can trap and accumulate smaller wood, branches, leaves and other organic materials that add to the complexity and diversity of instream fish habitats.”
LWD in Laramie River, with ice buildup.

Anchored rootwads encircled by ice.

During a recent project in the Laramie River, rootwads and boulders were added to banks and midstream to increase the quality and diversity of brown trout habitat.

Keller and Swanson noted that “once a tree falls into the channel it may reside there a long time and depending on the size of the stream and other factors may greatly affect channel form and process.”  So true.  I still have vivid memories of the Deep Hole and the Big Log in the creek by our family cabin.  Thanks to large woody debris, these were great places to skip rocks and watch water skippers, as well as spots where the trout gathered.
Dave fishing the Big Hole, 1966.
In the late 1960s heavy rain and high flow wiped out the Deep Hole and the Big Log.  Now, in the absence of large woody debris, there is only a shallow briskly-flowing creek.

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