Investigating Bluff Retreat on Lake Huron
by Bob Montgomery
Bluff Retreat is a significant concern in many parts of the Great Lakes. When and where it occurs, bluff retreat damages property and infrastructure, and recent research suggests that bluff retreat adds nitrogen and phosphorus to our lakes that contributes to harmful algal blooms.
In 2023, Ben Woodward completed an undergraduate thesis at the University of Waterloo, examining bluff retreat along the Lake Huron shoreline between Grand Bend and Goderich. One thing he found is that erosion is not equally distributed along the shoreline. “Actually on a lot of the shoreline, even during high water levels, the shoreline doesn't erode at all, or erodes very minimally. There's a couple of main areas that had notable erosion. One was Poplar Beach, which is just south of St. Joseph. That had the most dramatic erosion at approximately 1.2 metres per year on average and during high water years it would get up to almost three metres.” Woodward says there was also notable erosion just south of the harbour in Bayfield, and north of Bayfield on Melena Beach there has been some, but not much recently, and just south of Goderich there's been some major gullying in the bluffs. But, Woodward says other than that most of the bluff areas between Goderich and Grand Bend are quite stable.
Woodward also found that the speed at which water levels rise is a bigger factor than how much the levels rise. “It's true that when water levels are high there's more erosion because during periods of rising water the beach always adapts to whatever water level the lake is at, but there's a lag of a year or two for it to do that. So when the lake level is rising quite rapidly, that's when you're going to see the water going over top of the beach and starting to attack the bluff.” Woodward says one example of that is in the 1960's there was actually some erosion when the lake levels weren't at their highest but they were rising quite quickly from a very low period to average levels.
Woodward says climate change and wind are going to have a significant impact on erosion going forward. This year we had record low ice cover throughout the Great Lakes and some of the windiest months of the year are December and January and those are months when the shoreline is generally protected by the ice so it doesn't really matter how much the wind is blowing. But now, with climate change, the shoreline is being exposed during those months, so potentially, during the next period of high lake levels we could see even faster rates of erosion because the shoreline is exposed for more of the year.
Woodward adds, through their study they were able to identify several factors that may lead to greater bluff retreat at specific locations. They include:
Bluff material: Most of the bluffs between Grand Bend and Goderich are composed of silty clay till. This till was compacted under the weight of the Laurentide ice sheet during the last glaciation, making it stiff and resistant to retreat. This is one of the reasons why southeastern Lake Huron’s coastal bluffs erode more slowly than the sandier, less compacted ones found along the north shore of Lake Erie.
Wave energy, bathymetry, and substrate: Some parts of the lakebed near the shoreline have rocky bottoms due to an underlying stony till layer, which may dissipate wave energy.
Beach width, sediment supply, and shore structures: Wider beaches dissipate some or all the wave energy that would otherwise reach the bluff toe, protecting it from retreat. Beach width depends on sediment supply. Along some areas of the shoreline, sediment supply is restricted by human-built structures that jut out from the shoreline, such as piers and groynes.
They also found a fourth cause was agricultural drainage directly onto the bluff face. This is especially prominent at Poplar Beach, where several tile drains emit runoff directly onto the bluff face. This additional water moistens and weakens the bluff material, making it more vulnerable to slides and slumps.
Woodward will be presenting more on his study at the Bayfield Historical Society's Annual General Meeting. The meeting will be held on April 15th at 11:00am, at the Bayfield Lions Club Building located at 6 Municipal Road, Bayfield. No ticket or RSVP is required.
