Lake Superior level rose more than average last month

NEWS RELEASE

INTERNATIONAL LAKE SUPERIOR BOARD OF CONTROL

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The International Lake Superior Board of Control, under authority granted to it by the International Joint Commission (IJC), has set the Lake Superior outflow to 2,060 cubic meters per second (m3/s) (72.7 thousand cubic feet per second (tcfs)) for the month of May, effective May 2. The May outflow is 410 m3/s (14.5 tcfs) less than that prescribed by Regulation Plan 2012.

The Board recently requested and received approval from the IJC to temporarily deviate from Plan 2012. The outflow of Lake Superior will be adjusted over the next several months to accommodate expected maintenance at the hydropower plants, and reduce the potential for adverse consequences of high and fluctuating flows and water levels in the St. Marys Rapids. To maintain more consistent flow conditions in the rapids and minimize the effects on both Lake Superior and Lake Michigan-Huron water levels, the Board expects that flows less than those prescribed by Plan 2012 will be during periods of reduced hydropower flow capacity, and that these flow decreases will be offset by releasing flows greater than Plan 2012 during the months when no hydropower limitations are expected. At most, the differences in lake levels in any given month due to these changes are expected to be less than 2 cm (0.8 in) on either lake, and cumulative differences are expected to be negligible by the end of November.

The May outflow of 2,060 m3/s (72.7 tcfs) is expected to exceed the combined capacities of the hydropower plants on the St. Marys River, which will be approximately 1,656 m3/s (58.5 tcfs) in May, and most of the remaining flow will be released through the control structure at the head of the St. Marys Rapids. The gate setting of the control structure for the month of May will be maintained at the current setting equivalent to two gates open.

Based on provisional data, the net water supplies to Lake Superior were above average in April. The level of Lake Superior rose 15 cm (6 in) last month, while on average the lake rises 8 cm (3 in) in April. The Lake Superior level at the beginning of May is 21 cm (9 in) above average, 2 cm (0.8 in) above the level recorded a year ago at this time, and 32 cm (13 in) above its chart datum level. 

The net water supplies to Lake Michigan-Huron were above average in April. The level of Lake Michigan-Huron rose 18 cm (7 in) last month, while on average the lake rises 11 cm (4 in) in April. The level of Lake Michigan-Huron is 31 cm (13 in) above its long-term average beginning-of-May level, 6 cm (2 in) lower than it was a year ago, and 74 cm (29 in) above its chart datum level.

The levels of Lake Superior and Lake Michigan-Huron are both expected to continue their seasonal rise in May.

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