Over the summer months, Cleveland Water staff have been closely watching as the waters of Lake Erie separate into distinct layers – a layer of warm water at the surface and a cold water layer at the bottom of the lake.
Most recently, the temperature of surface water measured by our buoy 3.5 miles offshore is around 76°F while 51-feet below the surface the temperature drops to around 52°F. The warm and cold water layers are separated by a layer of rapid temperature change between 30 feet and 37 feet below the surface.
This layering process is called stratification. Lake Erie stratifies twice a year, summer and winter, forming distinct layers based on water density differences caused by temperature variations.
In winter, ice is the least dense water layer and floats on top, very cold dense water stays below the ice, and the warmest water is at the bottom of the lake.
In the summer, the opposite is true. Water warmed by the sun is less dense and floats on top. In the Central Basin of Lake Erie, the water is too deep for the sun to reach the bottom of the lake, so those waters stay cooler. By mid-summer, the density of the water layers are so different they stop mixing together.
When stratification occurs, there is a limited amount of dissolved oxygen in the cold bottom waters. Over time, this oxygen is slowly used by the decomposition of organic matter and respiration by animals and plants living there. When dissolved oxygen drops below levels that support normal aquatic life, the water is called hypoxic. When dissolved oxygen levels drop to zero, the waters are anoxic – or absent of dissolved oxygen.
Sometimes hypoxic water occurs in tiny pockets that form and move based on weather patterns. Other times, like during a hot, dry spell with very little wind, a large area of hypoxic water can form.
Hypoxic water doesn’t present a health or safety concern for drinking water, but it can cause taste and odor issues. We constantly monitor Lake Erie for hypoxia and other water quality parameters using data collected by our buoys as well as the system of buoys spread across the lake, sensors in our intake tunnels, and from NOAA’s Lake Erie Hypoxia Forecast Model. If we spot any issues, we can adjust our water treatment process so our customers don’t notice a difference in water coming from the tap.
The temperature graph above, taken from one of our buoys, shows how water in Lake Erie has stratified. Water at the surface (red line) is around 76°F while water near the bottom (grey line) is 24°F colder at 52°F. The water temperature changes rapidly between 30 feet (pink line) and 37 feet (black line) below the surface.