Study Explores Erosion of Protective Islands in Lake of the Woods

Extreme flooding coupled with human development appear to be primary factors contributing to ongoing erosion of barrier islands in Lake of the Woods.  

These islands help protect an ecologically sensitive bay in Minnesota and Ontario, according to a recent IJC-funded study. 

In 2022, the International Rainy-Lake of the Woods Watershed Board supported studying erosion on southern shore barrier islands in Lake of the Woods, consisting of Pine and Curry Island in the United States and Sable Islands in Canada. These islands serve as crucial habitat and protect the shoreline and ecosystem of Fourmile Bay from wave action.  

Since Western records first started being kept in the early 1800s, the islands have changed—notably when Norman Dam was constructed to the north of the lake and raised the water level. But the islands had existed in a quasi-stable state from the 1880s for roughly a century, said Zac Morris, a study lead and Coastal and Waterfronts Department manager with AMI Consulting of Superior, Wisconsin.  

Since about 1991, managers in the watershed noticed the islands have been eroding at an alarming rate, particularly Pine and Curry Island, which has lost about 1 mile (1.6 kilometers) of its length, Morris noted. 

“Without the extra mile (of the barrier islands) the waves have eroded away the (south shore of the lake) and aquatic vegetation,” Morris said. “Water quality and ecosystem health has been affected in an adverse way.  

“We’re losing a lot of submerged aquatic vegetation and fish habitat. Other issues include (impacts on) recreational economics, cultural value and the history of the islands themselves.” 

The board contracted AMI and engineering firm LimnoTech’s Oakdale, Minnesota, office to investigate causes of this erosion pertaining to the board’s responsibilities regarding water quality and ecosystem health in the watershed.  

AMI’s report identified five potential factors contributing to the erosion and narrowed in on high water levels and up-drift structures as those most likely to impact the barrier islands. 

To investigate, AMI collected information including survey data for Sable Islands, bathymetric data to study the topography underwater of the bay, wave measurements and sediment samples.  

AMI also used additional survey data for Pine and Curry Island collected by Lake of the Woods County.  

Similarly, researchers were able to reference relevant hydrological and bathymetric surveys by the US Geological Survey for the rest of Lake of the Woods and its connecting channels, and wind data from NASA. 

In partnership with AMI, LimnoTech leveraged the data to produce computer models to simulate different conditions in the lake, including degrees of water flow, winds and water levels based on historical measurements.  

The models found that much of the erosion seemed to take place during high flow and high wind events. In these instances, high water levels would cause breaches in the islands that would then continue to erode even when lake levels returned to average.  

“Reduced sediment supply from up-drift appears to have shifted the sediment balance for the islands to one of net loss,” said Craig Taylor, civil engineer with LimnoTech. “These combined effects suggest that the islands have lost their ability to self-heal.” Even sediment entering from the Rainy River appears to bypass Pine and Curry Island, Taylor added. 

Morris said the study found that likely culprits of the erosion were the northern jetty at Zippel Bay to the west of the islands, built in the mid-1980s by the US Army Corps of Engineers, as well as development and armoring of the shoreline of Lake of the Woods to the west by local property owners.  

Both changes appear to have disrupted the flow of sediments to replenish the barrier islands.  

Additionally, the county dredges the channel to Zippel Bay for small boat traffic and deposits that sediment out of the system. This also may be a factor, along with climate change and more extreme weather. 

The report recommends a second phase to the study, which would include collecting further data measuring sediment transportation, soil samples and profiling the sediment below the bottom of the lake to confirm the findings from existing models.  

“The overall goal of phase 2 is to validate our models and engage the stakeholders,” Morris said. “There are a lot of people with a great interest in this project, whether they’re interested from a tribal or cultural heritage perspective, are recreational boaters, or are from surrounding communities (or government agencies), we have to ensure all their voices are heard.” 

With more data and an engagement process, Morris said, this second phase could then develop a broadly acceptable approach to restoring the islands. A second phase proposal is being developed and finalized by the board and its partners. 

The IJC provided funding for this effort through its International Watersheds Initiative. Additional in-kind and financial support came through the Minnesota Department of Natural Resources, Lake of the Woods County, and Lake of the Woods Soil and Conservation District.