At Lake Springfield, full pool is the 560-foot mark. Above this point,
water will flow over the crest of the gates onto 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 to nine inches per month.
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 to four inches below full pool or lower, 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.
Whenever possible, CWLP Water Plant Operators seek 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 and wave damage. As a hedge against drought, the
return to summer operations with a target of full pool at 560.50 MSL, is planned to resume each spring.
The highest lake water level ever recorded 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. These pumps have since been abandoned and replaced to
the new Low Service Pump Station located northeast of the Lakeside Station to a more secure and less flood prone facility. The lowest water level
ever recorded was 547.44 feet, which occurred on December 29, 1954, during the drought of 1953-1955.
The return of heavy spring rains in 1955 prevented the lake level from dropping to 13 feet
below full pool at which power and water service to Springfield customers would have been disrupted
due to the location of the low service pumps at that time and the inability to draw in water.