Originally published in Lake of the Woods Area News, Volume 54, Number 5, Winter 2024
Bathymetry is the measurement of the depth of a water body and it tells us about the underwater profile of the bed of lakes and streams. This information helps us to understand aquatic habitats, how water flows, which areas might be most vulnerable to water levels changes, and how sediments and nutrients move through water.
Recently, the International Joint Commission (IJC) has funded two research projects through the International Rainy–Lake of the Woods Watershed Board (IRLWWB) that involved bathymetric surveys of parts of the watershed. AMI Consulting Engineers and LimnoTech worked on these studies.
In the first project, a survey of the lower Rainy River, in July 2024, the field crew used multibeam scanners to map the bottom of the entire width and length of the Rainy River from Manitou Rapids to Fourmile Bay (Figure 1). The information they have gathered will be used this winter to generate a model of water depths and flows for this segment of the river.
The second project, the Lake of the Woods Southern Shore Barrier Islands Erosion Investigation, was done in 2023.
Pine & Curry Island is in the U.S. and Sable Island is in Canada (Figure 2). Pine & Curry Island has experienced significant erosion at its southwest end since the 1960s (Figure 3).
The barrier islands provide protection to the south shore of Lake of the Woods, an area of sensitive shoreline that would otherwise be damaged by erosion from wind and waves. South shore erosion means loss of critical habitat and increases sediments and nutrients to the south basin of Lake of the Woods, where excess nutrients (phosphorus) already fuels nuisance algae blooms.
The study tested five hypotheses to explain the observed erosion. Is erosion of the barrier islands the result of:
- High lake levels and large wave events?
- The construction of the Norman Dam (1890s), which elevated the level of the lake?
- Interruption of sediment transport from nearby shorelines (i.e. there’s no material feeding the renewal of the islands)?
- Reduced sediment flows from the Rainy River?
- Changes (reduction) in ice cover due to climate change, resulting in more wind and wave exposure?
Over the summer of 2023, technicians took bathymetric measurements to map the underwater profile, from the relatively deeper water at Zippel Bay, around these barrier islands, through shallow Fourmile Bay. Underwater doppler acoustic sensors measured wave activity at a depth of 20 m. These data, plus historical information about water levels and island topography dating back to the 1890s, were shared with LimnoTech, a firm that used the information to create a model for how wind and waves operate on this part of the lake. The model simulates what happens to lake bottom sediments and the underwater portions of the barrier islands under different wind and wave conditions.
The study demonstrated that several things drive the loss of the barrier islands.
First, when the Norman Dam was constructed at Kenora, Lake of the Woods rose around three feet—enough to erode the original islands that lay at the mouth of the Rainy River, and to sweep in new sediment from nearby shorelines. These sediments became the barrier islands we know today.
In recent years, major high-water events (especially in 2014, 2022, and likely 2024) have, again, driven rapid erosion of parts of the current barrier islands. Once erosion has begun, the process continues even after water levels recede.
The recent high-water events resulted from increased heavy spring and summer precipitation, which is occurring more frequently as the climate warms. The heavy rain itself can also cause erosion. Ice cover is also diminishing due to climate change and an extended open water period allows for more seasonal erosion from heavy rainfall events, wind and waves.
Finally, in recent years, managers around Zippel Bay have tried to protect that shoreline from erosion by installing hard surfacing. While this has provided intended protection, it also interrupted an important source of sediment that formerly replenished Pine & Curry Island.
The final report from this project, including recommendations to governments for some prospective next steps to try to solve this complex challenge, is available on the IRLWWB website at ijc.org/en/rlwwb.
