Fluctuations in Lake Levels - types
Fluctuations in water levels in non-tidal areas are the result of several natural factors and may also be influenced by human activities. These factors operate on a time-scale that varies from hours to years. The levels of the Great Lakes depend on their storage capacity, outflow characteristics of the outlet channels, operating procedures of the regulatory structures, and the amount of water supply received by each lake. The primary natural factors affecting lake levels include precipitation on the lakes, run-off from the drainage basin, evaporation from the lake surface, inflow from upstream lakes, and outflow to the downstream lakes. Man-made factors include diversions into or out of the basin, consumption of water, dredging of outlet channels and the regulation of outflows.
There are three types of water level fluctuations on the Great Lakes:
Wind generated waves are superimposed on all three categories of water-level fluctuations.
These fluctuations result from persistent low or high net basin supplies. They result in extremely low levels such as were recorded on some lakes in 1926, the mid-1930s and mid-1960s, or in extremely high levels such as in 1952, 1973 and 1985-86. More than a century of records in the Great Lakes basin indicate no regular, predictable cycle. The intervals between periods of high and low levels and the length of such periods can vary widely and erratically over a number of years, and only some of the lakes may be affected. The maximum recorded range of monthly water-levels, from extreme high to extreme low, have varied from 1.2 metres for Lake Superior to over 1.8 metres for the other lakes. The ranges of levels on Lakes Michigan-Huron, Erie and Ontario reflect not only the fluctuation in supplies from their own basins, but also the fluctuations of the inflow from upstream lakes.
A plot of average annual water level recorded on each of the Great Lakes for the period 1918 to 2006.
These fluctuations of the Great Lakes levels reflect the annual hydrologic cycle. This is characterized by higher net basin supplies during the spring and early summer and lower net basin supplies during the remainder of the year. The maximum lake level usually occurs in June on Lakes Ontario and Erie, in July on Lakes Michigan-Huron, and in August on Lake Superior. The minimum lake level usually occurs in December on Lake Ontario, in February on Lakes Erie and Michigan-Huron, and in March on Lake Superior. Based on the monthly average water levels, the magnitudes of seasonal fluctuations are quite small, averaging about 0.4 metres on Lakes Superior, Michigan and Huron, about 0.5 metres on Lake Erie, and about 0.6 metres on Lake Ontario. However, in any one season it has varied from less than 0.2 metres to more than 0.6 metres on the upper lakes, from less than 0.3 metres to more than 0.8 metres on Lake Erie and from 0.22 metres to 1.10 metres on Lake Ontario.
A plot of the average (for the period 1918 to 2000 ) monthly mean water level shows the features mentioned above for each of the Great Lakes.
TThese fluctuations, lasting from a less than an hour to several days, are caused by meteorological conditions. The effect of wind and differences in barometric pressure over the lake surface create temporary imbalances in the water level at various locations. Storm surges are largest at the ends of an elongated basin, particularly when the long axis of the basin is aligned with the wind. In deep lakes such as Lake Ontario, the surge of water level rarely exceeds 0.5 metre, but in shallow Lake Erie, water-level differences from one end of the lake to the other of more than 5 metres have been observed. Although the range of fluctuations may be large, there are only minor changes in the volume of water in the lake. A seiche is the free oscillation of water in a closed or semi-closed basin; it is frequently observed in harbours, bays, lakes and in almost any distinct basin of moderate size.
A plot of hourly water level for three days, from December 11 to 13, 2000, at six gauging stations on the north shore of Lake Erie shows the water level fluctuations caused by storms. The Bar Point gauge is located at the mouth of the Detroit River, Kingsville is in the western basin, Erieau and Port Stanley are in the central basin, Port Dover and Port Colborne are in the eastern basin.
Superimposed on all three categories of water-level fluctuations are wind-induced waves. Surface waves can be a hazard to navigation and are also the main cause of shore erosion. Surface waves start small, but as they travel more or less downwind, the waves grow in height, become longer and move faster. Although the Great Lakes are large, the fetches they present to the winds ensure that the waves are under-developed (except in light winds) and tend to be steeper. Research has determined the following characteristics of waves on the Great Lakes:
- the closed boundaries effectively eliminate "swell" (long waves propagating from distant storms);
- when the fetch varies substantially about the wind direction, the largest waves tend to diverge from the wind direction towards the long-fetch direction;
- very under-developed waves move faster than fully-developed waves of the same length;
- the longest waves in an undeveloped sea are much steeper than their fully-developed counterparts.
[source for this information: "Thermal Structure and Circulation in the Great Lakes", F. M. Boyce et al, Atmosphere-Oceans, 27 (4) 1989, 607-642].
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