Dude, Where's My Beach?
As summer cottagers began their vacations, some had to wonder where their beach had gone. Particularly in dune areas, the shoreline appeared to be overtaken with dune grasses. What actually happened was a confluence of natural processes.
After Lake Huron’s last high lake level in 1997, water levels dropped dramatically in 1998 and stayed well below average for nearly a decade and a half. With the low levels came wide expansive beaches. These open sandy beaches were prone to wind erosion, and sand was blown and deposited into the dunes as part of their natural re-building process.
Marram grass, a dominant dune grass on Lake Huron beaches, has its growth rate stimulated by sand burial. The wide open beaches created an opportunity for the grass to grow lakeward. That’s a good thing. If it didn’t grow lakeward, causing the dune to grow wider, the dune would instead grow higher and people would be living beside a vertical wall of sand. Most dunes instead grow wider, toward the lake, building up the sand reserve that helps protect the shore and maintain good quality beaches once higher lake levels return.
Over the past one and a half years, lake levels have rebounded impressively, rising nearly a metre since the record low in 2013. The wet fall, winter and spring contributed to levels continuing to rise well into July.
(Lakeward dune migration, followed by rising lake levels, resulted in a temporary reduction in open beach space. As storm waves erode the dunes, the open beach will widen).
For cottagers, the timing was a bit off. The dunes that had grown lakeward for the past number of years was being met with a rising shoreline, leaving little open sand on which to lay a beach towel. But Mother Nature eventually puts things right. Open beaches are open because of an important process: storm waves. The reason we have open beaches at all is because of storm wave activity. If that were not the case, we would have forests growing right up to the water’s edge.
So the lack of beach in some areas this summer is a matter of timing. Our storm season is still ahead of us. The gales of November produce substantial storm wave energy that causes shore erosion, and the fall and winter storms should open up the beaches again. Unless, of course, lake levels keep rising. But then you’ll be glad you have those dunes for protection and for sand supply for your beach!
[article by Geoff Peach, Coastal Resources Manager]
Lake Turnover - a natural process, or a tasty dessert?
What is meant by "lake turnover"? How and why does Lake Huron do this in autumn and spring?
The key to these questions is how water density varies with water temperature. Water is most dense (heaviest) at 4º C and as temperature increases or decreases from that temperature, it becomes increasingly less dense (lighter). In summer and winter, lakes are maintained by climate in what is called a stratified (or layered) condition, where less dense water is at the surface and more dense water is near the bottom.
During late summer and autumn, air temperatures cool the surface water causing its density to increase. The heavier water sinks, forcing the lighter, less dense water to the surface. This continues until the water temperature at all depths reaches approximately 4º C. Because there is very little difference in density at this stage, the waters are easily mixed by the wind. The sinking action and mixing of the water by the wind results in the exchange of surface and bottom waters which is called "turnover."
During spring, the process reverses itself. This time ice melts, and surface waters warm and sink until the water temperature at all depths reaches approximately 4º C. The sinking combined with wind mixing causes spring "turnover."
The layering and turnover of water annually are important for water quality. Turnover is the main way in which oxygen-poor water in the deeper areas of the lakes can be mixed with surface water containing more dissolved oxygen. This prevents anoxia, or complete oxygen depletion, of the lower levels of most of the lakes. Which is good! However, the process of stratification during the summer also tends to restrict dilution of pollutants from effluents and land runoff. Which is not so good.
During the spring warming period, the rapidly warming nearshore waters are inhibited from moving to the open lake by a thermal bar, a sharp temperature gradient that prevents mixing until the sun warms the open lake surface waters or until the waters are mixed by storms. Because the thermal bar holds pollutants nearshore, they are not dispersed to the open waters and can become more concentrated within the nearshore areas.
(River plume in spring when river water is warm relative to the lake water. The warmer water 'rafts' over the colder, denser lake water. The dark brown 'tea-like' water in the river plume is caused by tannins from decaying leaves in the river valley. - LHCCC photo, June 8, 2010)
Scientists studying the effects of climate change on the Great Lakes are discovering that water temperatures of the Great Lakes are increasing because of the warmer summer air temperatures and longer ice-free season. Warmer waters could affect the timing and frequency of lake turnover. In a warmer climate, the turnover may not occur every year in all lakes. Turnover is the main way for deeper lake waters to become replenished with oxygen, and without enough dissolved oxygen, cold water lake fish and other species would be unable to survive in their deep water habitats.
More on Lake turnover - click here.
Have a Guilt-Free Labour Day
Ah, Labour Day. The last long weekend in the year when people go to the beach to forget about work. That may be great for most people. But at the Coastal Centre...the beach is where we work! That's OK. Don't feel guilty. We love what we do. However, if you're sitting on the beach, with a refreshing beverage, thinking, 'those poor super heroes at the Coastal Centre, working to make this beach I'm sitting on so awesome, what can I do to help them?' Well, be guilty no more! You can donate to the Centre...your local beach protectors!
You can donate online through CanadaHelps, or send a donation cheque by mail. For more information on donating to the Centre, visit our donate page.
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