Boulder South Lake Water Temperature
Boulder South Lake in Perry County, Illinois averages about 67°F at the surface in summer — warmest in Jul (~69°F) and coldest in Apr (~56°F). These are typical values by month, not a live reading.
Typical water temperature by month
Typical surface water temperature by month, from 347 field readings (2016–2021) via the EPA Water Quality Portal. Months with too few readings are left blank.
Is Boulder South Lake warm enough to swim?
Brisk but swimmable for short dips — most people acclimate after the first few minutes.
Want more than temperature? Boulder South Lake scored B 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 ~61°F
Summer average ~68°F
Summer average ~84°F
Summer average ~69°F
Summer average ~84°F
Summer average ~71°F
Explore water temperatures
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Frequently asked questions
What is the water temperature of Boulder South Lake?
Boulder South Lake has a typical summer surface temperature of about 67°F. It is usually warmest in July (around 69°F) and coldest in April (around 56°F). These are typical values by month from 347 EPA Water Quality Portal field readings (2016–2021), not a live reading.
When is Boulder South Lake warmest?
Surface water at Boulder South Lake is typically warmest in July, reaching about 69°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 Boulder South Lake warm enough to swim?
At its typical summer peak of about 67°F, Boulder South Lake rates as "cool" for swimming. Brisk but swimmable for short dips — most people acclimate after the first few minutes.
Where does this Boulder South 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 — 347 readings from 3 monitoring stations between 2016 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.