Spirit Reservoir Water Levels

The annual operating cycle for the reservoir is described below and illustrated by the following graph:

Spring - Snowmelt and spring rains generally produce the highest natural river flows of the year. To reduce downstream flood flows, as much water as possible is stored in the reservoir. In normal and wet years the water level will rise to the full mark by mid-April. In dry years the water level may only rise to within 1.5 feet of full.
Summer - Natural river flow drops dramatically as evaporation and transporation (water used by plants) reach their highest levels of the year. When the flow of the Wisconsin River at Merrill drops below the flow goal, water is released from the reservoir to augment the flow. In wet years the water level may only drop about 1 foot below full. In a normal year, the water level will start dropping in mid-June and reach 4 to 4.5 feet below full by late-August. In very dry years the water level may fall to 8 feet below full by the end of summer.

Fall - Evaporation and transpiration decline and fall rains generally produce significant runoff. In wet years there is enough runoff to completely refill the reservoir in fall. In normal years the water level will rise througout fall reaching within 1 foot of full by November. In dry years there may be no significant runoff in fall and water levels may remain 8 to 9 feet below full throughout fall.
Winter - Natural river flow is low because precipitation occurs in the form of snow. Water is released from the reservoir throughout winter to augment the low flow on the Wisconsin River and at the same time provide storage space for spring runoff. In wet years, the water level may only drop to about 5 feet below full by the end of winter. In a normal year, the winter drawdown will begin in late-November and the water level will drop steadily throughout winter reaching about 14 feet below full by mid-March. In very dry years the water level may reach the minimum level of about 17 feet below full by the end of winter.