Woody debris in urban watersheds - Not so cut and dried
Understanding the critical and beneficial role that woody debris plays in a river's ecology and how to properly manage it is crucial when dealing with large woody debris issues in urban watersheds.
Downed trees and other woody debris that accumulate in rivers are often believed to be a problem that needs to be addressed through removal. This is especially true in urban watersheds where woody debris and logjams are highly visible, collect trash, and are an obstacle for recreational uses within the river. However, woody debris such as fallen trees is a natural and important part of aquatic ecosystems that may provide more benefit than harm, and should be managed with this in mind.
A collection of trunks, limbs and branches is often referred to as woody debris. This debris accumulates within rivers and can range in size from a single floating stick all the way to a large log jam consisting of several full-sized tree trunks that span a river channel. These larger accumulations are often referred to as Large Woody Debris (LWD).
LWD can provide food and cover for fish and insects that live in lakes, rivers and streams. Wood and associated organic material (leaves, fungi, etc.) are an integral part of the food web which supports small aquatic species such as insects, which in turn provide food for fish and other larger animals. LWD can create desirable habitats for aquatic species, such as deeper sections of water called pools within the river, by slowing water down and redirecting flows. In addition, when properly managed, LWD can alter flows to provide erosion control and add physical structure to river banks and the river bottom.
In many cases, LWD in a river can and should be left alone because of the benefits that it provides. However, there are times when woody debris can have negative impacts on the river and action is needed. For example, LWD can block navigation, disrupt flow patterns which result in increased erosion, or pose a hazard or block structures such as culverts or bridges. In these situations, the woody debris needs to be actively managed. Four key points regarding the proper management of woody debris include:
- Minimize the removal of wood to maintain the benefits provided when addressing the issue or concern.
- Utilize the existing wood in ways that can be beneficial to the stream. This can include re-orienting the wood or anchoring it to the bank or within the channel. The orientation of the wood within a stream often influences how flow is altered and what changes may occur in the stream.
- In cases when removing wood from the river is necessary, place wood high enough and far enough away from river so it does not reenter with high flows.
- Be mindful of surrounding habitat and minimize disturbance of these areas while conducting needed maintenance.
Keep in mind that a permit is not required when dealing with floating debris and logs that are not embedded in the stream bottom or banks. However, in more extensive management scenarios, a permit from the Michigan Department of Environmental Quality may be required. Refer to the “Field Manual on Maintenance of Large Woody Debris for Municipal Operation and Maintenance Crews” for additional details on woody debris evaluations, management scenarios, recommendations, and permit requirements. This management manual was produced by partners within Michigan’s Clinton River Watershed.
Some additional questions which are critical to successful LWD management:
- Who has responsibility for managing LWD in our rivers and streams? The short answer is that the responsibility falls to the landowner; however, there is no requirement to actively manage woody debris in a certain way.
- What resources are available to help properly manage woody debris within the river? In some locations, communities and local units of government may help out with woody debris management or coordination of cleanups. Local organizations such as watershed councils will often assist with cleanups and can also help coordinate management efforts. In Southeast Michigan, the Clinton River Watershed Council is integrally involved in LWD management with many communities within the watershed.
Other helpful resources include the Woody Debris Management 101 and 201 documents produced by partners in Wayne County and the Rouge River watershed. These documents explain two different management methods: Clean and Open and Habitat Structure.
For information or questions regarding permitting for woody debris management, contact the Michigan Department of Environment Quality.