Natural factors affecting Great Lakes levels

Learn about the natural factors affecting the water levels in the Great Lakes.

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Precipitation and runoff

Precipitation is the source of all water reaching the Great Lakes. This is in the form of:

  • rain
  • snow
  • condensation

Over-lake precipitation represents a large and immediate supply of water to the Great Lakes. This is because about a third of the Great Lakes basin area is lake surface.


The amount of precipitation is fairly constant throughout the year. However, winter precipitation stored as snowpack is a major contributor to spring runoff to the lakes.

The land area that contributes runoff to the Great Lakes is a band 10 to 150 km wide around the lake shores. It’s drained by a system of rivers and intermittent streams.


On the Great Lakes, the average annual evaporation from the lake surface is almost equal to the average annual precipitation onto the lake surface.

Evaporation from the land and water surfaces depends on:

  • solar radiation
  • humidity and wind
  • temperature differences between the air mass and the water

Evaporation from the Great Lakes is:

  • greatest in the fall and early winter when the:
    • lakes are relatively warm
    • air above the lakes is cold and dry
  • least in the spring and early summer when the:
    • lakes are cold
    • air above the lakes is warm and moist

Condensation to the lake surface may result instead of evaporation.

Other natural factors

The Great Lakes water levels and outlet flows are also affected by:

  • groundwater, which is believed to be a minor component in adding or removing water from the lakes
  • ice retardation in the winter, which occurs when the flows in the outlet rivers are impeded by ice:
    • jams
    • formation
  • aquatic growth during the summer
  • tides, which are:
    • only a few centimetres in the Great Lakes
    • masked by larger fluctuations caused by meteorological disturbances
  • crustal uplift (isostatic rebound) since the last glaciation, which may:
    • have a long-term effect on lake levels
    • tilt the basin and/or change the elevation of the outlet channels

Meteorological disturbances

Meteorological disturbances are superimposed on the annual cycle of water levels and the multi-year fluctuation in supplies. They cause short-term fluctuations over time frames that can range from hours to days.

The change in water level caused by these disturbances may be more pronounced in certain parts of a lake. This is a result of:

  • shoaling water
  • funneling by shoreline configuration
  • gradually sloping inshore bottom, which reduces the reverse sub-surface flow

Atmospheric pressure

If there’s a difference in atmospheric pressure over a body of water, the water level will be:

  • lower under the area of high pressure
  • higher under the area of low pressure

In the absence of other forces, the water surface slopes to adjust to the differences in atmospheric pressure along the surface.


The slope of the water surface in the direction of the wind stress is wind set-up. The water level at the downwind end of the lake will rise.

The difference in water level between the 2 ends of the lake depends on the:

  • lake:
    • depth
    • shape
    • length
  • wind:
    • speed
    • duration
    • direction

The change in water level is greatest when a strong wind blows over a long, shallow lake for a long time.

Storm surges

Storm surges are pronounced increases in the water level associated with the passage of storms.

Most of the change is a direct result of atmospheric pressure and wind set-up. However, the storm travelling over the water surface can cause a long surface wave to travel with it to shore.