The Benefits Of Large Woody Debris

From Restoring The Au Sable, December 1996

In October, 1996, The Au Sable River Watershed Restoration Committee received a grant from the Michigan Habitat Improvement Fund to implement a demonstration project for placement of woody debris in the Au Sable River below the Mio impoundment. The Michigan Habitat Improvement Fund is a result of the FERC Re-licensing Agreement between Consumers Power Company, the Michigan Department of Natural Resources, the U.S. Forest Service, and other state and federal agencies. (The Anglers were very involved in this agreement as a member of the Michigan Hydro Relicensing Coalition.)

The $100,000 grant will be used to develop techniques for the placement of whole, large trees in the river for the purposes of habitat enhancement, food production, and erosion control. Other techniques, such as structure fabrication using smaller, more manageable material, will also be incorporated where necessary.

In the late 1800s and early 1900s, logging utilized the river for the transport of timber. This practice lead to the removal of deadfall trees which naturally fell into the river providing the necessary habitat for the native and now extinct grayling. Historical accounts indicate that, before the river log drives, the river was not navigable. Several areas of the river were so full of woody debris that the river actually seemed to disappear underground, according to these accounts. Native Americans used these logjams for trails to cross the river.

In the early to mid-1900s, hydroelectric dams were installed to harness the river's energy. These dams interrupted the natural river with impoundments, warming the water and making the river unsuitable for many native fishes. Dams also reduced or eliminated natural migration of fish, especially those from the Great Lakes which rely on stream habitat for spawning. Large scale land clearance and development have also changed the landscape of the river corridor.

Scientists now know that logjams and debris complexes are vital for proper functioning of biological components within a stream. They know that physical aspects of the river strongly influence the biological components. Fallen trees alter the flow of stream current which is important not only for fish but also for all the other organisms which fish rely on for forage. Organisms seek out areas of slower current for resting; without this refuge, energy is needlessly spent and survival is precarious. Fallen trees also deflect current away from highly unstable streambanks. Submerged trees help the river's current to scour deep holes used by fish for refuge and cover.

Large deadfalls trap smaller debris, slowing the transport of organic material that is vital to the ecological balance of stream organisms. Streams rely on the same sources of energy as terrestrial systems. Sunlight, through photosynthesis, produces energy in the form of algae, which is the fundamental building block of the food chain. Another source of energy is from the terrestrial environment itself through the input of organic material such as leaves, grasses, and woody twigs. Streams which are shaded by trees along the bank rely heavily on this external source of organic material to the food chain.

In the Au Sable, aquatic insect larvae and other invertebrate organisms are highly dependent on this source of energy for survival and in turn create food for fish.

A leaf falling into a stream is immediately colonized by bacteria and fungi. Leaves are food for those aquatic insects, including some species of mayflies and caddisflies, sometimes called "shredders," that process the leaf by chewing, utilizing the fungi and bacteria for growth. Fecal material--ground leaves passed through the shredders--is then utilized by the "collector" aquatic insects, and they in turn process this material even more for use by other "collectors." Predatory insect larvae, such as some stoneflies and dragonflies, prey upon these smaller insect larvae. Fish at various stages of their lives utilize all of these insects to sustain life and grow.

Woody debris is also food for those organisms that burrow into the tree to process the fibrous woody tissue. But more importantly it acts to trap leaves both whole and processed to be used by other insects. It is this function, as a retention device, that keeps the organic material from being transported downstream and hence not properly processed and used to its fullest extent.

The benefits of large woody debris are many. Increased cover, habitat variety, feeding stations for fish, and homes for fish and insects are examples of physical benefits. Biological benefits include increased forage production and species diversity providing more food in a variety of forms that fish need throughout their different life stages.

Fallen trees are as natural to our rivers as the trees that shade them. We are slowly realizing that in order to restore a stream's fishing potential we need to restore its habitat potential, not just for the fish but for the insects that give them life.RWOL

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