Grand Lake St Marys — Current Water Temperature, Annual Trends and Forecast
This page provides information on the current water temperature in Grand Lake St Marys, trends, monthly ranges and the forecast for the coming days.
Current Water Temperature
Water Temperature in Grand Lake St Marys today is 20.6°C.
Water Temperature: Daily Trends, Historical Records, and Forecast
✅ As of today, the minimum water temperature ranges from 68°F (20°C) to 74°F (23°C). The water is warm and pleasant for swimming, and in this range, one can stay in the water for extended periods without the risk of hypothermia. This is the ideal temperature for relaxation, swimming, and water sports. Families with children, as well as outdoor enthusiasts, will enjoy swimming and time spent in the water. For most people, this is the optimal temperature for a comfortable beach experience. It is important to note that these swimming tips and recommendations apply only in good weather. When the air temperature is below 68°F (20°C), and sometimes even below 77°F (25°C), swimming can be unpleasant even with relatively high water temperatures.
Water Temperature Chart Over the Last 30 Days
Table of Daily Sea Temperature Data
| Date | Current | Average | Forecast |
|---|---|---|---|
| JUN 3 | 18.1°C | 20.7°C | |
| JUN 4 | 19.6°C | 20.7°C | |
| JUN 5 | 19.8°C | 20.7°C | |
| JUN 6 | 20.4°C | 20.7°C | |
| JUN 7 | 20.8°C | 20.7°C | |
| JUN 8 | 20.2°C | 20.7°C | |
| JUN 11 | 20.7°C | 20.6°C | |
| JUN 12 | 20.7°C | 20.6°C | |
| JUN 13 | 20.7°C | 20.6°C | |
| JUN 14 | 20.7°C | 20.6°C | |
| JUN 15 | 20.7°C | 20.6°C | |
| JUN 16 | 20.7°C | 20.6°C | |
| JUN 17 | 20.7°C | 20.6°C |
* Current - Actual water temperature value
** Average - Average water temperature on this day in recent years
*** Forecast - Our water temperature forecast
Actual nearshore water temperatures may fluctuate by several degrees from the reported values, especially following heavy rainfall or extended periods of strong winds. Certain wind patterns can cause colder, deeper waters to rise and replace the sun-warmed surface water, leading to noticeable variations.
Our forecast is based on a proprietary mathematical model that considers real-time water temperature changes, historical trends, key weather patterns, wind strength and direction, and air temperature specific to each region. Additionally, we incorporate data from other resorts to enhance accuracy.
Monthly Water Temperature Range (Min, Max & Average)
| Month | Min | Avg | Max |
|---|---|---|---|
| January | 1°C | 4°C | 6°C |
| February | 0°C | 2°C | 5°C |
| March | 0°C | 5°C | 11°C |
| April | 6°C | 12°C | 18°C |
| May | 13°C | 17°C | 19°C |
| June | 17°C | 21°C | 24°C |
| July | 20°C | 23°C | 25°C |
| August | 23°C | 26°C | 28°C |
| September | 19°C | 24°C | 28°C |
| October | 15°C | 20°C | 24°C |
| November | 6°C | 11°C | 15°C |
| December | 1°C | 5°C | 9°C |
Grand Lake St Marys: Water Temperature by Location
Grand Lake St Marys: Weather Forecast
June 9
June 10
June 11
June 12
June 13
Location & Local Information
Grand Lake St. Marys is a relatively shallow reservoir in western Ohio, so its surface water temperature responds quickly to seasonal weather. In winter the lake often sits near freezing, roughly 32–40°F (0–4°C), and can develop ice in cold spells. Spring warming typically brings surface readings into the mid 40s to low 60s °F (7–16°C) as air temperatures rise. Summer sees the warmest conditions, with surface temperatures commonly in the 70s to low 80s °F (21–28°C) on warm days because the shallow basin heats quickly. In autumn the water cools back through the 50s to mid 60s °F (10–18°C) before approaching winter lows again. Actual temperatures vary day to day with wind, sun and rainfall, and deeper spots (though limited) remain cooler.
Swimming is permitted in designated park beaches and public access areas when local authorities judge water quality to be safe, but the lake routinely experiences nutrient-driven algal blooms and occasional bacterial issues that can prompt advisories or temporary closures. Because the shallow water warms readily, conditions that favor blue‑green algal growth (cyanobacteria) are common in summer, and officials monitor sites and post warnings to protect public health. Visitors should follow posted notices and avoid swimming during visible scums, discoloration, or posted advisories.