Pond Turnover Risk Index

Understanding the Turnover Risk Index

The Natural Waterscapes Turnover Risk Index (TRI) is a proprietary forecasting tool that analyzes weather conditions to predict the likelihood of pond and lake turnover events at the county level across the contiguous United States. It's the first tool of its kind available to the public.

Turnover events are one of the most common — and preventable — causes of fish kills in ponds and small lakes. By monitoring your county's TRI rating, you can take proactive steps to protect your aquatic ecosystem before dangerous conditions develop.

What Causes Pond Turnover?

During warm months, ponds naturally develop thermal stratification — warm, oxygen-rich water sits on top while cooler, oxygen-poor water settles to the bottom. These layers remain stable as long as temperature gradients are maintained.

When air temperatures shift rapidly — such as a sudden cold front in late summer or an unseasonably warm spell in early spring — the surface water temperature can change quickly enough to destabilize this layering. The water "turns over," mixing the oxygen-depleted bottom water throughout the pond and causing a sudden drop in dissolved oxygen across the entire water column.

High winds compound the problem by physically mixing the water layers. Heavy rainfall can also contribute by adding large volumes of cold water to the surface.

How the TRI Works

The TRI integrates data from the National Weather Service to calculate a composite risk score (0–100) for every county in the contiguous 48 states. The model evaluates multiple factors including the rate and magnitude of temperature change over a 48-hour window, wind speed and duration, precipitation volume, proximity to the critical 39°F density inversion point, and accumulated freezing degree days for ice cover estimation.

Risk calculations are depth-zone specific. Shallow ponds (under 6 feet) respond to temperature changes more quickly but are less likely to develop severe stratification. Deep ponds (18+ feet) are more resistant to rapid mixing but face greater oxygen deficits in their bottom waters when turnover does occur.

Depth Zone Selection

Use the depth zone selector above the map to match your pond's maximum depth. The risk score changes significantly between depth zones because the physics of thermal stratification and mixing are different at each depth range.

Shallow (under 6 ft): These ponds mix more frequently and are less likely to develop dangerous stratification. However, they're more vulnerable to rapid temperature swings and have less thermal mass to buffer oxygen levels.

Moderate (6–18 ft): The most common depth range for farm ponds and recreational ponds. These ponds develop meaningful stratification during summer months and are the most susceptible to classic turnover events.

Deep (18+ ft): Lakes and deep ponds with significant stratification. When these water bodies turn over, the volume of oxygen-depleted water mixing into the upper column can be substantial.

How to Respond to Elevated Risk

When the TRI shows elevated or higher risk for your area, here are the steps you should take.

If you have aeration installed: Make sure your system is running 24/7. This is not the time for timer schedules. Continuous aeration is your best defense against turnover-related oxygen crashes.

If you don't have aeration: Consider installing a surface aerator or fountain immediately. For ponds under 5 feet deep, surface aerators like the Kasco 2400AF or 3400AF can rapidly boost dissolved oxygen. For deeper ponds, a bottom-diffused system provides the most comprehensive protection.

Monitor your fish: Watch for signs of stress — fish gasping at the surface, congregating near inflows, or showing unusual lethargy. These behaviors often precede a major oxygen event.

Why Aeration is the Best Prevention

A properly sized aeration system prevents dangerous turnover by continuously mixing the water column and maintaining consistent dissolved oxygen levels from top to bottom. Aerated ponds don't develop the extreme stratification that makes turnover dangerous — they stay mixed, oxygenated, and safe for aquatic life year-round.

Learn more about the different types of aeration systems and how to choose the right one for your pond in our comprehensive aeration guide.

NOAA Satellite Ice & Snow Overlay

The satellite toggle above the map activates NOAA's Interactive Multisensor Snow and Ice Mapping System (IMS), showing real-time ice and snow cover at 1km resolution. When ice cover is detected over a county, the TRI model accounts for this — ice-covered ponds are not susceptible to wind-driven turnover, but the transition from ice-covered to open water (ice-off) can trigger a mixing event.