Why do Great Lakes water levels usually rise in summer and fall in winter?

The water levels of the Great Lakes fluctuate throughout the year due to the balance between water inputs, such as precipitation, and outputs, primarily evaporation.1

Generally, lake levels rise to their highest points in mid-summer and fall to their lowest levels in late winter. This seasonal pattern is driven by meteorological factors:2

• Summer Rises: Higher water levels often coincide with or follow seasons of increased precipitation. Additionally, the release of water from winter snowpack during the spring thaw contributes to rising levels as the year progresses.1
• Winter Falls: Lower water levels typically coincide with seasons of high evaporation. During the colder months, the lakes lose significant amounts of water to the atmosphere through evaporation, which is a primary output factor for the hydrologic balance of the system.13

While these seasonal variations are common, the specific timing and magnitude of these fluctuations can vary from year to year based on exceptional periods of precipitation or drought.1