Natural Factors Affecting Lake Levels
The natural factors that affect water level fluctuations include: precipitation, evaporation, runoff, groundwater, ice retardation, aquatic growth, meteorological disturbances, tides, crustal movements and meteorological disturbances.
Precipitation in the form of rain, snow and condensation is the source of all waters reaching the Great Lakes. Over-lake precipitation represents a large and immediate supply of water to the Great Lakes because about one third of the Great Lakes basin area is lake surface. The land area contributing runoff to the Great Lakes, in a band from about 10 to 150 km wide around the lake shores, is drained by a system of rivers and intermittent streams. The amount of precipitation is fairly constant throughout the year, but winter precipitation stored as snowpack is a major contributor to spring runoff to the lakes.
Evaporation from the land and water surfaces depends on solar radiation, on temperature differences between the air mass and the water, and on humidity and wind. Evaporation from the Great Lakes is greatest in the fall and early winter when the air above the lakes is cold and dry and the lakes are relatively warm. Conversely, the evaporation is least in the spring and early summer when the air above the lakes is warm and moist and the lakes are cold. Condensation to the lake surface may result instead of evaporation. On the Great Lakes, the average annual evaporation from the lake surface is almost equivalent to the average annual precipitation onto the lake surface.
Groundwater is believed to be a minor component in adding or removing water from the lakes.
Ice retardation in the winter, when the flows in the outlet rivers of the Great Lakes are often impeded by ice formation or ice jams, and aquatic growth during the summer also have an effect on outlet flows and hence lake levels.
Tides, which are the periodic rise and fall of the water resulting from the gravitational interactions of the sun, moon, and earth, are only a few centimetres in the Great Lakes and are masked by larger fluctuations caused by meteorological disturbances.
Crustal uplift (isostatic rebound) since the last glaciation may tilt the basin and/or change the elevation of the outlet channels and have a long-term effect on lake levels.
Superimposed on this annual cycle of water levels and the multi-year fluctuation in supplies are meteorological disturbances causing short-term fluctuations over time frames ranging from hours to days. If there is a difference in atmospheric pressure over a body of water, the water level will be lower under the area of high pressure and 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 term wind set-up refers to the slope of the water surface in the direction of the wind stress; the water level at the downwind end of the lake will rise. The difference in water level between the two ends of the lake depends on the length, shape and depth of the lake and the duration, direction and speed of the wind; the change in water level is greatest when a strong wind blows over a long, shallow lake for a long time. Storm surges are pronounced increases in the water level associated with the passage of storms. Although most of the change is a direct result of atmospheric pressure and wind set-up, the storm traveling over the water surface can cause a long surface wave to travel with it. The change in water level caused by these disturbances may be more pronounced in certain parts of a lake as a result of shoaling water, of funneling by shoreline configuration or of a gradually sloping inshore bottom which reduces the reverse sub-surface flow.
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