Layers in Ambergate Reservoir: Stratification and Turnover

Ambergate Reservoir is a fictional lake that supplies a town with drinking water. It is also home to fish. In summer, the water does not mix from top to bottom. Instead, it settles into layers. These layers make summer a difficult season for both the fish and the water plant.

The sun warms the top of the reservoir. Warm water is lighter than cold water. So the warm water floats near the surface. The cold, heavier water sinks and stays near the bottom. Between the warm top and the cold bottom lies a middle layer called the thermocline. In the thermocline, the temperature drops quickly over a short distance. This layered pattern is called stratification.

The layers do not mix in summer. The thermocline acts like a lid. It keeps the top water and the bottom water apart. Air touches only the surface. So only the top layer can pick up oxygen from the air and from tiny water plants. The bottom water is sealed off below the thermocline.

Oxygen matters because fish need it to breathe. Most fish need dissolved oxygen above 5 mg/L. Cold water can actually hold more oxygen than warm water. But holding capacity is not the same as supply. Down in the dark bottom water, dead plants and animals sink and rot. Bacteria use up oxygen as they break this matter down. Because the bottom is cut off, that oxygen is never replaced. Over the summer, the deep water slowly loses its oxygen.

The table shows a late-summer profile of the fictional Ambergate Reservoir.

The table shows the problem for a fish. Near the surface, there is plenty of oxygen, but the water is warm. Deep down, the water is cool, but there is almost no oxygen. A fish that wants both cool water and enough oxygen can get squeezed into a thin middle band. Sometimes it finds no comfortable layer at all.

The layers do not last forever. In the fall, the air cools. The surface water cools too. As it cools, the top water grows heavier. When the top water becomes as heavy as the bottom water, the lid is gone. Wind can then push the whole reservoir into motion. The water mixes from top to bottom. This yearly mixing is called fall turnover.

Turnover is good news for the deep water. Mixing carries oxygen from the surface down to the bottom. The fish that were trapped between bad layers get relief. But turnover can also stir up mud and odd smells from the bottom. For a few days, the water plant may notice a strange taste or color in the water it pulls from the reservoir.

Operators plan around these changes. Many reservoirs have intake pipes at more than one depth. In late summer, an operator may skip the low-oxygen bottom water and draw from a higher, fresher layer. Understanding the layers helps keep both the fish and the tap water healthy.

1. In summer, what happens to the water temperature from the surface down to the bottom of Ambergate Reservoir?
2. Using the table, at which depth does the dissolved oxygen first drop below the 5 mg/L that most fish need?
3. What is the thermocline, and what does it do to the reservoir's layers?
4. Why does the deep water lose its oxygen over the summer, even though the surface water has plenty?
5. The passage says cold water can hold more oxygen than warm water. Yet the cold bottom water has the least oxygen. Explain why.
6. What is fall turnover, and why does it help the fish that live in the deep water?
7. In late summer, an operator draws water from a higher layer instead of from the bottom. Based on the passage, why is that a good choice?
8. Using the table and the passage, explain why a fish that needs cool water and oxygen above 5 mg/L might struggle in late summer.