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The Critical Role of Wood in Michigan Rivers Taking Action on the Upper Manistee
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The Critical Role of Wood in Michigan
Rivers: Taking Action on the Upper Manistee
by Kristin Thomas, Michigan TU Aquatic Ecologist
The role of wood in Michigan rivers is an often discussed and debated topic among anglers and scientists alike. One thing is sure; wood is critical to the morphological and biological health of Michigan rivers. The morphological importance is especially pronounced in the sandy, low-gradient streams found in the Lower Peninsula.
There is consensus that rivers were filled with wood before we chopped down all the trees and cleared the rivers for log drives—much more than we see in streams today. Clearcutting and driving logs not only removed all the wood in our rivers but also removed the source of new wood, interrupting recruitment for years to come. In addition, humans have continued to remove wood from streams and riparian areas, furthering the interruption in recruitment. The result is many streams which would benefit from more wood and more consistent recruitment of new wood.
Wood is critical in Michigan streams not only as habitat for fish and aquatic insects, but also to the morphology or shape of the river. The structure that wood provides in our slow, sandy streams is what creates diversity of habitat. As water flows into a downed tree, it is forced to change direction and speed. These changes in the flow of water create deep pockets, areas of hard substrate, and slow areas for deposits of sand and silt. Imagine a large tree has fallen into a stream you love to fish. The area where the tree fell was wide, sandy, and shallow. Typically, you would pass right by, thinking no fish here. But what do you think when you come to fish and see the new tree? Perhaps you think about the new pocket of deep water that will form downstream of that tree as water flows quickly over the tree, churning and moving sand on the downstream side. Maybe you also see slow water upstream of the tree and think of a new spot for sand, or perhaps silt to settle out—new Hex habitat.
You might also look at the tip of the tree and see a bubble line. Notice that water is being pushed to the middle of the stream. Maybe that increase in velocity will move some sand and uncover gravel. The concentration of flow might create more depth towards the middle of the stream as well. You may also think of big fish hiding under the new structure, eating insects that colonize the wood, and enjoying the shade, deep water, and overhead cover. One big tree—all this habitat diversity. When these low gradients, sand-bed rivers lose their wood, and they lose their diversity. Water velocity is slow and monotonous, sand settles out, depth becomes uniform, and diversity decreases. We don’t have the big floods of higher gradient systems to flush out sand and create new habitats. Without these flushing flows, Michigan streams have a hard time repairing themselves. This is the scenario observed in much of the Upper Manistee River between Yellow Trees and CCC Bridge, especially between Yellow Trees and Rogers Landing. Wide, shallow, sandy stretches of the river. Corner pools that are deep but have minimal structure or cover. Old CCC structures were no longer doing their intended job. Every project partner who floated this stretch agreed there was not one spot that had too much wood. An increase would be of benefit just about everywhere. And so, a project was formed: a multi-tiered approach to adding wood to the Upper Manistee River. One of the project’s goals is to use a variety of techniques – hinge cuts, single whole trees, and groups of whole trees – to see which techniques work best. Which provide biological and morphological diversity? Which maintain themselves over time and look natural through time? Which will translate well to other low gradient sandy Michigan systems? A project with Upper Manistee River hinge cuts, crisscrossed in pools to create diverse cover and habitat.
a purpose greater than itself; improve the Upper Manistee but also work together as a team to determine in a broader context what works well for similar rivers. This means monitoring is a big piece of the project. The only way to see what happens after the wood is placed is to monitor depth, substrate, river shape, and macroinvertebrates to see how things change.
The various treatments are being implemented in phases. Phase one was to partially cut riparian trees to fall in the river for habitat, also known as hinge cuts. In most cases, the trees create cover and diversity of velocity in corner pools, providing places for big fish to hide and eat. In some cases, trees were cut to push water towards the middle of the river in wide shallow areas to mimic that big fallen tree we talked about earlier. Creating a place for big fish and bugs to live, the slow water upstream might become colonized with aquatic vegetation and create excellent shallow water habitat for juvenile fish. In all cases, the goal was an increase in habitat diversity. The hinge cutting technique was chosen for various reasons: the area is hard to access, and bringing trees in overland would be difficult, expensive, and disruptive. Other groups had been experimenting with hinge cuts with success. It is a “natural” method for adding wood mimicking the natural order of things; trees grow tall and old, water moves along the bank, and eventually, they fall in the stream.
Hinge-cutting work was completed in August. In total, 135 trees were cut into the river between Yellow Trees and CCC Bridge. Many were red pines located in corner pools. These trees were cut to add habitat diversity for pools in the form of cover for fish, habitat for macroinvertebrates, and variety in the velocity and direction of flowing water. To maximize cover and secure trees in place, they were often crisscrossed on top of each other, both to create a tangled mess of branches for fish and bugs to live in and to secure them in place. There will be future work to build up the inside of these bends to narrow the stream and provide additional depth in some locations.
There were also several long, wide, shallow runs where trees were cut on alternating sides to push water towards the middle of the stream. These trees were angled to provide a location for sand and silt to deposit upstream, create small pockets of deep water downstream, and push water towards the middle of the river to develop a more consolidated channel. The objectives were to add physical habitat for fish and macroinvertebrates while creating a diversity of velocity and pockets of deep water and providing areas for fine substrate to deposit. The next phase in these areas is to add whole trees to the stream channel to diversify flow further and provide cover and habitat. These trees will be placed by a helicopter drop in the fall of 2022.
In the meantime, monitoring of the hinge cuts has begun. Assessment of each area where trees were cut has been conducted, which includes noting information about the bottom substrate, water depth, structure stability, substrate deposition, aquatic vegetation, and most importantly, taking photographs. Rapids assessments will be conducted for the next three years to track changes in the river and structures. We will be watching for positive changes expected in the form of habitat diversity and also looking out for any unexpected negative changes. The only way to improve upon our methods and the associated outcomes is to monitor with open minds. It is hoped a few of you will conduct some hook and line surveys in the treatment areas and let us know what you find.
This work was identified and completed through the Upper Manistee Collaborative. Funding was provided by a Michigan Department of Natural Resources Fisheries Habitat Grant and through Great Lakes Restoration Initiative Funds granted by the National Fish and Wildlife Foundation.
The Legacy of a River is an amazing gift…
