Flood of 1993 took unique course in Mason County, Illinois

By Wendy Martin
Havana, Illinois

Hundreds of homes were lost in the Great Flood of 1993, and damages totaled more than $15 billion. It was one of the most significant and damaging natural disasters ever to hit the United States.

While river communities up and down the Mississippi and the Illinois rivers found themselves inundated in 1993, it was interior portions of Mason County that were underwater.

Mason County is unique among the more than 100 counties in the state of Illinois. While the Illinois River, which forms the county’s western boundary, was way over its banks, the flooding that occurred in 1993 came from the bottom up, miles from the river.

Whole subdivisions found themselves with water up to their basement ceiling rafters as the underground aquifer filled - and overflowed. As a result of the flooding in 1993 which extended into 1994, a variety of studies were initiated, including some by hydrologist Gary Clark. Among the things he investigated were the nature and capacity of the aquifer underlying Mason County.

The aquifer is a thick layer of sand that extends across Mason County and into a portion of neighboring Tazewell County. The aquifer ranges in depth from 50 feet to 170 feet. In most places it is 90- to 100- feet deep.

According to Clark, the flood of 1993 was a 250-year event.

One of the effects were the appearance of 171 groundwater lakes covering 9,000 acres. One of the largest ones was Sand Lake, covering more than 500 acres. “In 1993 the ground water went up, but in most cases no more than 10 or 12 feet. But you have a shallow water table, so you got lakes and flooding,” Clark explained.

One question raised at the time was how did the Illinois River influence what was happening.

“The Illinois River was at flood stage and we wondered if that was one of the reasons we had problems with groundwater lakes,” Clark said. According to his findings the high river caused water to back up and become three to seven feet higher at the river’s edge, but five miles away from the river there was very little impact.

(A version of this story appeared in the Mason County (IL) Democrat. It is reused with permission.)