Apple turnovers. Yum. Your favorite basketball team with too many turnovers. Yuck. Reservoir turnover … wait, what?

Yes, reservoir turnover is a thing all its own. (No apples or basketballs are involved.)

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Reservoir turnover is a natural phenomenon often seen in late autumn and spring, when the water in lakes and reservoirs can “turn over,” or move from top to bottom and bottom to top due to temperature changes.

“The basic idea is the bottom layer goes to the top, top goes to the bottom,” explained Travis Bray, an aquatic species expert and part of Denver Water’s Environmental Planning team. “It happens most typically in the spring and again in the fall as the top layer warms and later cools.”

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At times, this churn can bring sediment up from the depths. On occasion, anglers or hikers might notice more cloudy water near the surface and perhaps see it as a troubling sign. 

In fact, nothing is “wrong” with the water; a natural process is underway.

Also, at this time of year, some residents may express concerns about the way some water tastes or smells. This, too, is connected to the lake’s turnover. 

Water churned up from the bottom is often depleted of oxygen. The low oxygen level affects water chemistry in a way that generates a richer nutrient mix. That water, when stirred up to the top of the reservoir, allows for conditions that support algae blooms. Those blooms, in turn, can affect the taste and odor of the water.

It’s important to emphasize: Even during this time, the water Denver Water delivers is safe to drink.

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For more about what turnover is, and why it’s important to the health of a lake, here’s an eloquent description from the Clean Lakes Alliance: 

“Twice a year, unseen forces churn water from the depths of our deeper lakes and deliver oxygen and nutrients essential to aquatic life. This temperature-driven process of lake ‘turnover’ allows aquatic life to inhabit the entirety of the lake as oxygen becomes more available. Without this natural process, our lakes would not be the vibrant and dynamic ecosystems we see today.”

Simply put, temperature change warms or cools water in the lake; those temperature changes affect the density of the water, bringing about movement in the water column that breaks up the water layers that form in the summer and winter. This creates a mixing of layers, or “turnover,” in the water.

Put yet another way, steadier temperatures in the summer and winter keep the water stratified. In the spring and fall, as air temperatures cool or warm, the water temperature also changes, creating more churn, or turnover, in the water.

For Denver Water, this pattern is potentially an issue at Marston Reservoir in southwest Denver, though study is ongoing to determine whether turnover plays a significant role there, or if there could be other factors at play. Fortunately, treatment experts at the utility are accustomed to this seasonal rhythm and can reduce the water taste impacts through treatment and by combining different water sources, or by bypassing Marston Lake altogether during turnover events. 

And while treatment plants can remove the offending algae, some residuals inevitably remain in the water, leading to small — and short-lived — impacts to taste and odor.

How do treatment plants work? Watch this video to learn more: 

That’s because some species of algae and bacteria naturally produce odorous chemicals inside their cells. For example, geosmin and MIB are common odorous chemicals.

The earthy and musty odors generated by Geosmin and MIB are detectable by many people at incredibly small concentrations of 5 to 10 parts per trillion (for a sense of how small that is: one part per trillion is one droplet in 20 Olympic-sized swimming pools). Hence the challenge in wholly eliminating the impacts of turnover through treatment.

Through the fall, as colder winter temperatures set in, the mixing in reservoir reduces as the water settles into more predictable layers — until wind, sunlight and higher temperatures set off another round of churn in the spring. Then, steadier temperatures through the summer settles the water into its layers again.

Alex Bentz, a water quality and treatment lead for Denver Water, said the utility is fortunate because its high-mountain water and mostly high-elevation reservoirs reduce the impacts of reservoir stratification, and ultimately, turnover. The natural process can be a far bigger challenge for utilities that depend heavily on lower elevation reservoirs, where turnover impacts can last longer and have bigger effects on water quality.

“Some Front Range utilities face huge challenges in this area,” Bentz said. “We just don’t run into situations where the water quality impacts (due to reservoir turnover) are so severe that we lose out on treatability of our water supply. We’re fortunate to be positioned to manage through these brief episodes.”

Learn more about where Denver’s water comes from in this video: 

All in all, it makes sense to reframe the cloudy or musty water as not only natural, but as beneficial to the health of the reservoir. The last word goes to Robert Ladwig, a hydrologist at the University of Wisconsin-Madison’s Center for Limnology (a fancy name for the study of lakes), courtesy of the Clear Lakes Alliance.

“I think of turnover as the lake taking a deep breath as everything is mixed,” Ladwig said. “It’s like a fresh start every spring and fall.”