Forbes Lake Water Temperature
Forbes Lake in Marion County, Illinois averages about 80°F at the surface in summer — warmest in Aug (~82°F) and coldest in May (~62°F). These are typical values by month, not a live reading.
Typical water temperature by month
Typical surface water temperature by month, from 106 field readings (2011–2021) via the EPA Water Quality Portal. Months with too few readings are left blank.
Is Forbes Lake warm enough to swim?
Very comfortable for swimming. Warm surface water can also favor blue-green algae in nutrient-rich lakes, so check for advisories before you go in.
Want more than temperature? Forbes Lake scored D on water quality — clarity, algae, phosphorus, depth, fish species, and swimming safety, all on one report card.
Water temperatures at nearby Illinois lakes
Summer average ~74°F
Summer average ~77°F
Summer average ~81°F
Summer average ~85°F
Summer average ~79°F
Summer average ~81°F
Explore water temperatures
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Frequently asked questions
What is the water temperature of Forbes Lake?
Forbes Lake has a typical summer surface temperature of about 80°F. It is usually warmest in August (around 82°F) and coldest in May (around 62°F). These are typical values by month from 106 EPA Water Quality Portal field readings (2011–2021), not a live reading.
When is Forbes Lake warmest?
Surface water at Forbes Lake is typically warmest in August, reaching about 82°F. Lake surface temperature lags air temperature by several weeks, so the warmest water usually arrives in mid-to-late summer even though the hottest air can come earlier.
Is Forbes Lake warm enough to swim?
At its typical summer peak of about 80°F, Forbes Lake rates as "warm" for swimming. Very comfortable for swimming. Warm surface water can also favor blue-green algae in nutrient-rich lakes, so check for advisories before you go in.
Where does this Forbes Lake water temperature data come from?
Every temperature on this page is a typical (climatological) value computed from real surface-water field readings collected by state and federal agencies and published through the EPA Water Quality Portal — 106 readings from 1 monitoring station between 2011 and 2021. They describe what is typical for each month, not the temperature right now.
About this data
Water temperatures are a climatology — the typical value for each month, averaged from real surface-water field readings collected by state and federal agencies and published through the EPA Water Quality Portal. They tell you what to expect, not the temperature at this moment. Actual conditions vary with recent weather, depth, and time of day.