How Lake Levels Are Maintained

Full pool is the 560-foot mark, at which point water will flow into the dam spillway even if no dam gates are lowered.

Average lake elevations vary according to the season. Highest average lake levels typically occur in the spring after the seasonal rains. The lowest average elevations occur in the fall and early winter months after a long, hot summer. For example, average October elevations are more than two feet below full pool.

It simply has to rain "normally" in the lake's watershed for Lake Springfield's water levels to remain at or above their monthly or seasonal averages. During hot, dry spells, however, Sugar and Lick Creeks—which are the primary sources feeding the lake—can dry up into a series of disconnected pools and contribute little or no water to the lake. In a typical summer, naturally low levels of rainfall, combined with high demand and increased rates of evaporation, can cause the lake's level to drop at a rate of about six inches per month.

This is a photo of the South Fork Channel Dam raised to cause water to back up into a nearby channel.

This is a photo of the South Fork pumping station, showing the channel into which water from the South Fork collects. The caption reads, Water backs up into this channel from which the pumping station can transfer it to the lake.

To supplement the supply available from Sugar and Lick Creeks, CWLP constructed a dam and pumping station in the South Fork of the Sangamon River in the 1950s. When the lake level drops below its average in any given month, the utility can—providing there is a sufficient flow rate in the South Fork—raise the dam, causing water from the South Fork to back up into a channel from which the water can be pumped into the lake.

As a hedge against the possibility of a spring drought, the utility tries, whenever possible, to maintain the lake's winter level at no less than six inches below full pool. In winter, keeping the level a little lower than 560 feet helps protect docks and other facilities from the potential for ice damage.

Excessively high or low lake levels can threaten power and water plant operations. When the water is too high, there is the danger it will flood the low-service pumps located in the sub-basement of Lakeside Power Station. If this were to occur, these pumps—which draw water into both the power and water plants—would be put out of commission. If the lake water level were to drop to 13 feet below full pool, it would be too low to reach the low-service pumps, thus putting the power and water plants out of commission. The highest lake water level ever recorded at Lake Springfield since it was first filled in 1935 occurred during a storm event on April 12, 1994, when the lake crested at 564.5 feet. Intensive sandbagging efforts at the Lakeside Power Station helped protect the low-service pumps from being flooded. The lowest water level
ever recorded was 547.44 feet, which occurred on December 29, 1954, during the drought of 1953-55. The return of heavy spring rains in 1955 prevented the lake level from dropping to the magic level of 13 feet below full pool at which power and water service to Springfield customers would have been disrupted.