Straight from the Tap

First Days of Summer on Lake Erie are Stratifying


Last week while monitoring sensors on Cleveland Water’s two buoys in Lake Erie, our staff noticed a downwelling event. Water temperature shifts like this can result in adjustments to the treatment process at our plants in order to maintain the quality and consistency of the water coming from your tap.

This week our source water is back to her normal summer self, stratifying. This means the water is separating into distinct layers based on water density differences. The water density differences are caused by temperature variations. Cold, dense water sinks to the bottom and stays there. Warm water is not as dense, so it floats on top.  

As viewed in the graph below, temperatures this week from the lake’s surface (red line) down to about 20 feet deep (light blue line) have mostly fluctuated between 68°F and 71°F, with a surface spike last weekend and drop to around 65°F yesterday. Drop five more feet down to 25 feet below the water surface (blue line) and the temperature is a few degrees colder but predominantly mimics the high and low swings of the surface water temperature.

But between 25 feet and 35 feet below the surface (top grey line), the temperature quickly drops to the mid- to low-50s. This area of rapid temperature change is called the thermocline. Water above the thermocline is warmer. Water below it is much colder. In fact, for the last few days, from 35 feet below the surface all the way down to 51 feet below the surface (bottom grey line), Lake Erie is downright chilly hovering around 48°F.

The cold water near the bottom of the lake is where our intake cribs, located 3 to 5 miles offshore, bring in water to send to our four treatment plants. Staff at the treatment plants keep close eyes on the temperature and other parameters of the water, such as pH and dissolved oxygen. They use this data to determine what adjustments to each plant's water treatment process may be necessary so we can deliver the highest quality water to your tap.

While our water treatment plant operators use buoy data to bring you the highest quality drinking water, you can also tap into data from buoys across the Great Lakes to help you on your next freshwater adventure, whether it's catching the big fish or winning the regatta. To find the data, go to

Red Line = Surface to 1 foot

Light Blue Line = 20 feet below the surface

Blue Line = 25 feet below the surface

Top Grey Line = 35 feet below the surface

Bottom Grey Line = 51 feet below the surface