Riparian vegetation, the plants that grow on the edges of streams (which I wrote about last week), serves many functions in aquatic ecosystems – plant roots stop riverbanks from eroding, grass and leaves shade the water and keep it cool, and limbs and trunks of trees fall into streams and create habitat and food for fish and insects.
Those limbs and trunks are collectively called ‘large woody debris’ by the scientists who study them. Two studies recently published in the Canadian Journal of Fisheries and Aquatic Sciences and the journal Water Resources Research examined the dynamics of how woody debris enters a stream, and how pieces of it move once they’re there.
A team of researchers from West Virginia University surveyed 25 headwater streams in West Virginia, both before and after Hurricane Sandy struck the eastern seaboard in Oct. 2012. They found that the effects of the storm were variable – in some streams, the level of woody debris didn’t change after Hurricane Sandy hit, but in others the scientists found almost three times as much wood after the storm. They also found that large wood inputs to some streams remained high for a year following the storm, perhaps because some trees or branches were weakened by the hurricane but didn’t immediately succumb to the effects of wind and snow.
A different team of scientists from the University of Southampton nailed an aluminum tag with a unique identification number onto each piece of large woody debris they found within five study reaches on the Highland Water, a river within the New Forest National Park in the U.K. They visited the study reaches several times over 32 months, and during that time 75 percent of the tagged pieces of wood moved; one log traveled three and a half miles from its original location.
The researchers found that logjams were a particularly important feature of the large woody debris dynamics in the river; where logs were already collected, more pieces of wood would tend to accumulate, often in the same locations during different seasons. Large logs seemed to anchor logjams, which smaller pieces of wood would cycle through, staying at one jam for a time before a flood would push them downstream to the next build-up.
In their paper, the scientists from the University of Southampton describe some of the many ways that wood and logjams influence streams and rivers: they can change water flow and sediment build-up patterns, alter the effects of floods by dissipating their energy, and create a variety of areas diverse in water depth, velocity, and gravel size, “which in turn provides habitat and refuges for a variety of aquatic and terrestrial organisms.”
Large woody debris can have large impacts on aquatic ecosystems; scientists are still sorting out the routes by which branches, twigs, sticks and logs make their way into streams, as well as their travels once they’re there.
Sources: Andrew RG & Hartman KJ. (2015) Uneven inputs of woody debris to Appalachian streams from superstorm Sandy. Canadian Journal of Fisheries and Aquatic Sciences 72(1): 1-6. doi: 10.1139/cjfas-2014-0208