Lake Decatur — Current Water Temperature, Annual Trends and Forecast
This page provides information on the current water temperature in Lake Decatur, trends, monthly ranges and the forecast for the coming days.
Current Water Temperature
Water Temperature in Lake Decatur today is 17.8°C.
Water Temperature: Daily Trends, Historical Records, and Forecast
⚠️ As of today, the minimum water temperature ranges from 60°F (15°C) to 65°F (18°C). The water is comfortable for most people, making it an ideal range for water sports enthusiasts. The temperature does not cause significant discomfort, and swimming will be pleasant for the majority. However, it's important to remember that prolonged exposure to the water can still lead to cooling, so it's essential to monitor the time spent in the water, especially for children or elderly people. 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 |
|---|---|---|---|
| OCT 5 | 20.5°C | 17.0°C | |
| OCT 6 | 20.7°C | 17.0°C | |
| OCT 7 | 20.1°C | 17.0°C | |
| OCT 8 | 19.0°C | 17.0°C | |
| OCT 9 | 19.2°C | 17.0°C | |
| OCT 10 | 18.8°C | 17.0°C | |
| OCT 11 | 18.5°C | 17.0°C | |
| OCT 12 | 18.6°C | 17.0°C | |
| OCT 13 | 17.0°C | 17.8°C | |
| OCT 14 | 17.0°C | 17.7°C | |
| OCT 15 | 17.0°C | 17.7°C | |
| OCT 16 | 17.0°C | 17.6°C | |
| OCT 17 | 17.0°C | 17.6°C | |
| OCT 18 | 17.0°C | 17.6°C | |
| OCT 19 | 17.0°C | 17.5°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 | 0°C | 2°C | 5°C |
| February | 0°C | 3°C | 7°C |
| March | 0°C | 6°C | 13°C |
| April | 8°C | 13°C | 19°C |
| May | 13°C | 18°C | 23°C |
| June | 20°C | 23°C | 26°C |
| July | 21°C | 26°C | 31°C |
| August | 23°C | 26°C | 29°C |
| September | 21°C | 24°C | 27°C |
| October | 13°C | 17°C | 21°C |
| November | 5°C | 10°C | 14°C |
| December | 0°C | 3°C | 6°C |
Lake Decatur: Water Temperature by Location
Lake Decatur: Weather Forecast
October 13
October 14
October 15
October 16
October 17
October 18
Location & Local Information
Lake Decatur is a man-made reservoir in central Illinois that experiences the full range of Midwestern seasonal temperatures. Surface water typically drops to near freezing in winter, often around 32–40°F (0–4°C), while spring sees a gradual rise into roughly 40–65°F (4–18°C). In summer, surface temperatures commonly reach about 70–85°F (21–29°C) during warm spells, though heatwaves can push readings a few degrees higher. By autumn the water cools again into the 50–70°F (10–21°C) band. Because the lake is relatively shallow in places and subject to wind mixing and inflows from the Sangamon River, deeper water can be notably colder than the surface, and temperatures can vary substantially from day to day.
Swimming at Lake Decatur is limited compared with coastal or purpose-built recreational beaches. The city and park areas generally promote boating and fishing, and there are few formal public swim beaches; this reflects concerns about water quality, occasional algal blooms, turbidity, and safety around boat traffic rather than a fixed statewide ban. As a result, swimming is often discouraged in many parts of the reservoir; anyone considering entering the water should check current local advisories, use designated recreation areas if available, and be mindful of changing conditions.