The Superior Rivers Watershed Association is pleased to issue our seventh edition of Marengo Riffles.
A Look at Natural Flood Management in the Marengo River Watershed: 10 Years of Recovery and Lessons Learned from the 2016 Flood
By Kevin Brewster
The July, 2016 flood event was a catastrophic moment that deeply impacted our communities, reshaped our landscape, and has changed how we approach regional flood preparation and prevention. For eight hours on July 11, 2016, a “train” of heavy thunderstorms dropped 8–10 inches of rain, with dramatically higher local values reported. In 15 hours, a US Geological Survey (USGS) stream gauge on the Bad River near Odanah recorded a flow volume increase from a normal 300 cubic feet per second to 40,000 cubic feet per second. Although centered on Ashland, Bayfield, and Iron Counties, the flood impact was felt across eight counties, resulting in three fatalities and loss of over $41 million in private property and public infrastructure-roads, culverts, power transmission lines, and more. It also vividly demonstrated how our communities pulled together on an unprecedented scale in mobilizing to care for flood victims. Unfortunately, this event was also an example of failure of federal disaster response policy to support local government efforts to replace lost road infrastructure with larger, more flood resilient installations. An opportunity for a coordinated regional effort to end years of road and culvert washout and replacement cycles was largely missed.
In this article, we will explore how this event-and a similar flooding disaster in 2018-have started reshaping how our communities respond to the threat of flooding. Federal, tribal and state natural resource agencies, environmental groups like SRWA and the Wisconsin Wetlands Association, and county-level land and water conservation and emergency response departments are increasingly focusing flood preparation and resilience strategies on natural flood management, or NFM. NFM can be thought of as any practice that prevents or reduces the impact of flooding by harnessing natural processes and landscape features-also known as ecological services. These practices can be relatively inexpensive activities like planting trees and shrubs in erosion-prone steep upper-watershed areas, stream floodplains, or in idle agricultural field drainage ditch systems. They can also be larger in scale and more expensive, such as engineered river bank stabilization projects or use of heavy equipment in reconnection of deeply-eroded stream channels with natural floodplains and nearby wetlands.
Examples of Projects in the Marengo Watershed
The East Altamont Road River Bluff Restoration
In Lincoln Township, East Altamont Road passes the Marengo River closely on a sharp bend in the river. The road is located along the edge of a steep hillside above the river channel known as a river bluff due to steepness caused by repeated landslides. The river current carries a lot of energy into a sharp right turn at the foot of the hillside. Because the bluff is largely composed of sand, it is easily eroded from above by rain and undercut from beneath by the strong river currents, resulting in a continuous process of river bank collapse as the channel wears deeper and deeper into the hillside. The road is an important lifeline for Lincoln Township-a critical access route for first responders, school buses, and connection to the greater community. Years of adding road fill and rocks to the base of the bluff only slowed the loss of the river bluff below the road, creating an endless expense and repair cycle. The flood of 2016 caused a major bank collapse, forcing road closure. SRWA, Bayfield County, and Lincoln Township had been discussing possible solutions to the problem for months prior to the flood event, and had settled on a plan for a major stream bank restoration project to save the road and finally control the ongoing erosion and sedimentation that were destroying an important cold-water fishery and important habitat for many other species. Because the Marengo River was being affected in close proximity to the Chequamegon-Nicolet National Forest, the project was eligible for funding through the USDA Forest Service Regional Advisory Committee (RAC) grants program.
The completed project reinforces and protects 200’ of streambank. The base of the bank, known as the bank toe, was excavated and reinforced with a latticework of logs tied together with stainless steel cable. Each large square cell of the lattice was filled with several feet of course gravel. The steep bank was recontoured with excavators and bulldozers to reduce the slope angle. The new bank edge was reinforced with installation of large tree root balls at the water’s edge. These provided roughness that slowed passing water, allowing suspended sediment to settle and the capture floating debris. This also created calm water zones for small fish and aquatic insects to thrive in. The entire bank area was then covered with topsoil and seeded with soil-stabilizing grasses. Follow up site work included planting of native willows, alders, and other shrubs.
Nine years later, looking at the restored river bank it would be easy to assume there never were erosion and bank collapse issues here. While a lot of engineering and machine work went into the project, the long-term healing process that it allowed to take place is very much in the realm of natural flood management. The stabilized bank was able to finally support grasses, shrubs, and young trees whose root systems continue to build the bank’s erosion resistance. The root balls at the water’s edge continue to reduce water velocity and capture sediment and debris. Over the years, these processes have built up a more erosion-resistant bank toe and are gradually pushing the channel away from the bank. The quiet water zones in and around the substantial collected woody debris have become safe microhabitat for aquatic insect larvae and other species (macroinvertebrates) that comprise the base of a food chain that nourishes trout, other fish, and much more. Young fish and larval amphibians also rely on these zones for feeding and protection from strong currents and predation.
Tody Ravine Stream Floodplain Reconnection
A very widespread erosion problem in our region is what is called stream incision, where high-energy surface water flows from paved areas, ditched agricultural fields, and deforested areas races into streams, causing increasingly deeper cutting of sandy or gravel channel bottoms. When incision becomes severe enough, the stream channel is too deep to allow natural stream overflow to dissipate energy and allow absorption of flood water across a complex, vegetation-rich floodplain surface. Instead, high flows from heavy storms or spring thaw race down the deep channel in a “firehose” effect, cutting the channel ever deeper and transferring high hydraulic energy to a constriction point like a culvert or sharp bend in the channel, resulting in a culvert washout or stream bank erosion and collapse. To bring natural flood control services back into the equation, re-connection of incised stream channels with their floodplains is clearly part of the solution, but other solution strategies are often evident. Examples include re-connection of streams to nearby wetlands, restoring upland tree and shrub growth, and blocking and replanting ditch networks in unused agricultural fields.
Tody Creek and is a relatively short reach of a deeply-incisced Marengo River tributary that crosses under State Highway 112 in White River Township. As commonly seen across our region, the continuous erosional downcutting of the stream created a ravine that is gradually migrating upstream as soil is carried away downstream. The site was chosen for a pilot study of the effectiveness of using post-assisted log structures (PALs) to raise water levels in the stream, reconnecting it to its floodplain. PALs basically mimic the effect of beaver dams in drainage areas, but in a more focal and controllable way. The structures slow water flow down, causing it to deposit sediment and debris, gradually decreasing the channel depth. In October, 2023 nine structures were installed, along with two rock riffle zones to also help slow water speed. Funding was provided by the 2019 WI Act 157 to Ashland County.
As the project comes up on three years old, the need for a few refinements have emerged. One issue that appeared immediately in the first spring on the site was that flows could be so high that they pushed around the ends of the PALs and carved channels through the stream bank, preventing the water from disbursing outward across the floodplain as desired. This was happening to the first several upstream structures that were exposed to the highest flow energy. There was evidence of floodplain inundation lower in the system with less by-passing of the structures. To remedy this, Ashland County Land and Water Conservation Dept., SRWA staff and volunteers planted numerous ground-stabilizing native shrubs in the streambanks near the PALs, and woody debris was added to reinforce some of the more flow-impacted structures. Red osier dogwood and willow chutes were also later planted across parts of the floodplain. The effect of the PALs is becoming noticeable with streambed levels rising as sediment is stopped and allowed to settle behind the structures. More floodplain water pooling is being observed, and a shift toward wetland vegetation is being noticed in the floodplain. Overall, the structures have remained stable and are continuing to hold back coarse sediment materials.
As the lessons learned in the Marengo Watershed continue to be applied in various ways, efforts are now underway to use a geographic information system (GIS)-based erosion and flooding hazard identification and prioritization toolbox developed for the Marengo River watershed further afield in the Lake Superior basin. The current project-a four year long initiative funded by the National Oceanic and Atmospheric Administration’s (NOAA) Climate Resilience Regional Challenge grant, involves SRWA, the Northwest Wisconsin Regional Planning Commission, Wisconsin Wetlands Association (WWA), Trout Unlimited (TU), the Burke Center, and others in expanding implementation of NFM strategies to four additional watersheds-the White River, Fish Creek, Montreal River, and Nemadji River watersheds. These watersheds are now undergoing rating for flood and erosion vulnerability and potential for application of NFM strategies. Local township boards and road crews have been joining WWA, SRWA, and Burke Center staff in ongoing problem site mapping sessions and field visits to identify and prioritize future restoration and flood resilience projects.
From our friends at the Lake Superior Collaborative:
Marengo Watershed Hydrologic Restoration
Restoring headwater streams and wetlands to reduce flooding
By Melinda Knutson
What does it take to restore headwater streams and wetlands to reduce flooding, erosion, and culvert failures downstream? How much funding and coordination is necessary, and what are the barriers? In 2020, the Ashland County Land and Water Conservation Department (LWCD) designed the Act 157 Marengo River Natural Flood Management project to answer these questions.
The Marengo River Watershed was chosen due to the availability of repetitive-loss culvert and road-stream crossing data, stream hazard vulnerability information, frequently reoccurring large floods, and major soil erosion events. The combination of steep terrain, erodible soils, historic and current hydrologic alterations, and multiple severe storms led to repetitive flood damage to road-stream crossings and farmlands. Numerous private landowners in the watershed have been adversely affected by these problems, and many are interested in cooperating with the LWCD to find solutions. The Marengo River is a tributary of the Bad River, which flows into Lake Superior.
Planning for Act 157 required the specialized expertise of multiple government agencies and technical contractors. It also relied on existing data sets and hydrologic models for the Marengo Watershed. Understanding and predicting where water will flow and how it will behave after high rainfall events is a complex undertaking! However, these predictions are essential to prioritize where and what restoration practices will be most effective and efficient. Ashland County LWCD was awarded funds from the WI Legislature through Act 157, and several state and federal grants. They assembled a skilled design team that included professionals and affected landowners, and developed plans for hydrological restorations at five properties within the watershed.
After years of conceptual planning, strict permitting, and design, implementation began at the first site in 2023. A combination of low-tech and traditional engineering practices was selected to ‘slow the flow’ of water by dissipating energy and connecting waterways back to their floodplains. At the Tody Ravine, eleven channel-spanning structures known as post-assisted log structures (PALS) and two Newberry Rock Riffles were installed to slow water and deposit sediment to promote the rising of the streambed elevation to reconnect the stream to its adjacent floodplain. At the Berweger Farm, severe gully erosion was restored through a wetland restoration, grassed waterway, rock Zuni Bowl, and a rock-lined channel. Downstream, at the Fischbach Wetland, a 10-acre wet site, was restored via wetland scrapes, a berm, and a rock spillway.
Permitting and regulatory barriers arose at two sites. These prohibited the projects from being implemented. The Anderson Farm & Berweger Road Culvert design included installing a larger culvert, restoring the stream to a two-stage channel, and stabilizing gullies and road ditches. The site is in a FEMA-mapped Zone AE floodplain, which added costly and time-consuming regulatory requirements that Ashland County did not have the resources to address. These rendered the project unviable within the budget and grant timeframe. This was disappointing because the FEMA maps were outdated, and there were administrative barriers to using more recent and accurate data. In fact, Ashland County could not find a contractor willing to take on the work due to past experiences of delays and project uncertainties when working in FEMA-regulated floodplains. The result is continuing non-point pollution, water quality impairment, high flood risk, and productive farmland continues to erode.
Regulatory barriers to management also developed at the Mackey Wetland. This site was identified as critical for hydrologic restoration because the large wetland is located at the bottom of the catchment, where the capacity to manage and store flood flows is essential. An embankment that sustained the wetland and protected the Marengo River Road from flooding was breached during the 2016 flood. It will likely fail in a future high rainfall event. As with the Anderson Farm & Berweger Culvert project, the Mackey site also lies within a Zone AE floodplain. As such, it is subject to the same costly and time-consuming regulatory requirements. In addition, the fix was interpreted as meeting the definition of a “dam” for regulatory purposes and would likely be denied a permit under current rules. Again, no action has been taken here to mitigate future habitat degradation and flood risk.
What Did We Learn?
The project highlighted the need for a shared vision among conservation partners to reap catchment-scale benefits. On the positive side, multiple agencies, led by Ashland County LWCD, worked together to successfully implement a variety of ‘low-tech’ solutions that can be implemented in the headwaters to reduce downstream flooding. Implementing hydrologic restoration at a catchment scale can leverage funding and amplify benefits but it also increases complexity. The project team needed to manage a higher volume of programs, grants, and regulatory coordination, uncovering a need for more planning and administrative resources for such large-scale efforts.
On the negative side, lower in the watershed, regulatory barriers were encountered when federal rules required additional modeling and permitting that was beyond the capacity and hydrologic modeling expertise of the partners. These rules may be necessary when dealing with the higher energy and more destructive potential of higher-order streams. However, more damage will be done if regulations prevent positive actions to mitigate flood risks.
What Are the Solutions?
- Sustain and increase funding for nature-based solutions to reduce flood risks. Funding agencies should incorporate timelines with more flexibility to meet additional regulatory requirements, if needed.
- The state should invest in baseline data and technical staff skilled in hydrologic & hydraulic modeling to support watershed-scale restorations, including vulnerability assessments and engineering design.
- Develop streamlined approaches for multi-agency coordination on complex hydrologic restoration projects.
- Review current regulatory requirements that affect watershed restorations and update policies regarding the intensity and cost of pre-planning.
- Update the FEMA maps. Federal agencies should keep these maps updated to reflect current conditions in the watershed.
- If restoration work in higher-order streams and their floodplains requires more intensive planning and engineering to meet regulatory requirements, the federal government should provide the technical expertise and underwrite the cost of this additional work in a timely manner. Delays increase the risk of catastrophic failure of key infrastructure.
