Lake Merwin — Current Water Temperature, Annual Trends and Forecast
This page provides information on the current water temperature in Lake Merwin, trends, monthly ranges and the forecast for the coming days.
Current Water Temperature
Water Temperature in Lake Merwin today is 18.4°C.
Water Temperature: Daily Trends, Historical Records, and Forecast
⚠️ As of today, the minimum water temperature ranges from 65°F (18°C) to 68°F (20°C). This is already quite warm water, which is ideal for most people. Swimming becomes comfortable, and prolonged stays in the water do not cause discomfort. This temperature is perfect for family trips and for those who prefer warmer water. It is also comfortable for water sports such as surfing or swimming. 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 |
|---|---|---|---|
| SEP 13 | 19.3°C | 20.1°C | |
| SEP 14 | 19.3°C | 20.1°C | |
| SEP 15 | 19.0°C | 20.1°C | |
| SEP 16 | 18.9°C | 20.1°C | |
| SEP 17 | 18.1°C | 20.1°C | |
| SEP 18 | 17.8°C | 20.1°C | |
| SEP 19 | 18.5°C | 20.1°C | |
| SEP 20 | 18.8°C | 20.1°C | |
| SEP 21 | 20.1°C | 18.5°C | |
| SEP 22 | 20.1°C | 18.6°C | |
| SEP 23 | 20.1°C | 18.7°C | |
| SEP 24 | 20.1°C | 18.7°C | |
| SEP 25 | 20.1°C | 18.8°C | |
| SEP 26 | 20.1°C | 18.9°C | |
| SEP 27 | 20.1°C | 19.0°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 | 3°C | 7°C |
| February | 0°C | 4°C | 8°C |
| March | 1°C | 5°C | 9°C |
| April | 5°C | 10°C | 15°C |
| May | 10°C | 16°C | 21°C |
| June | 15°C | 19°C | 23°C |
| July | 17°C | 22°C | 26°C |
| August | 21°C | 24°C | 27°C |
| September | 18°C | 20°C | 23°C |
| October | 10°C | 14°C | 18°C |
| November | 4°C | 8°C | 12°C |
| December | 3°C | 7°C | 10°C |
Lake Merwin: Water Temperature by Location
Lake Merwin: Weather Forecast
September 22
September 23
September 24
September 25
September 26
Location & Local Information
Lake Merwin, a reservoir on the Lewis River in southwestern Washington, experiences cool to moderately warm surface temperatures that vary by season and depth. In winter months the surface typically sits around 40–45°F (4–7°C). Spring sees a gradual rise, with surface waters commonly between 45–60°F (7–16°C) as the sun strengthens and inflows warm. Peak summer surface temperatures in July and August often reach roughly 60–72°F (15–22°C) in sheltered, shallow areas, while deeper water remains much colder. In autumn the surface cools back toward the 50–60°F (10–16°C) range. Because Merwin is a reservoir, thermal layering and colder deep water are common year-round; even in summer, sudden drops in temperature can occur below the surface.
Swimming at Lake Merwin is generally permitted and the lake is used for recreational swimming, boating and other water activities in designated shore areas and from boats, but there are no widespread lifeguarded beaches and rules or seasonal advisories may apply. The combination of cold water, variable currents near inflows and dam structures, boat traffic, and sudden depth changes means swimmers should take precautions: enter in daylight, wear a life jacket if uncertain, and be mindful of cold-water shock and hypothermia risk. Local signage and management agencies provide the best current guidance on where and when swimming is recommended.