Dam the Sangamon!

Driving across the Sangamon River at Reas Bridge Road, my first thought was, “They were going to dam the Sangamon here?”  My second thought was, “Just where the heck is the Sangamon River anyway?”

‘Here’ was on a bluff by an old cemetery just north of Reas Bridge Road overlooking the Sangamon River, also known in these parts as Lake Decatur.

As I looked out over the north end of Lake Decatur, it occurred to that sometimes the largest and proudest monument to a monumental effort is something you don’t even see. This is the story of this unseen monument to that monumental effort.

Lake Decatur was constructed in 1922 as a water source for Decatur and to supply water for one of its largest companies at the time, A.E. Staley. Building a dam in the main channel of the Sangamon River (as opposed to a tributary) was something of a risk due to the possibility of heavy sedimentation. So Lake Decatur was also considered to be an experiment at the time to determine the feasibility for building dams in flat agricultural areas.

The heavy sedimentation on the Sangamon River is a result of agricultural runoff. To reduce sedimentation, farmers were frequently urged to use soil conservation methods to keep topsoil from rushing off of their farmland and into the Sangamon River. 

Nonetheless, between 1922 and 1936, Lake Decatur lost 14% of its storage capacity to sedimentation. By 1946, a survey of sedimentation in Lake Decatur revealed that it was filling with silt at the alarming rate of 236 acre feet per year.

And by the mid 1960’s, another sedimentation survey of Lake Decatur revealed that it had lost 35% of its storage capacity due to siltation and had become laced with farm chemicals.

At this point, you or I might quite logically conclude that the Lake Decatur dam experiment was a failure and that building dams in flat agricultural areas was probably not such a good idea after all.

But to government officials, the solution to a failing dam seemed obvious: build another dam!

For years, the Army Corps of Engineers had been planning “Oakley Dam” on the Sangamon River near the rural town of Oakley for flood control purposes. With Decatur joining in support, Oakley Dam was authorized by Congress in 1962.

Along with the political powers in Decatur, the project was enthusiastically supported over the years by U.S. Senators Everett Dirksen, Charles Percy, Adlai Stevenson and Paul Douglas. Area Republican Congressmen William Springer and Edward Madigan were also supporters along with a succession of Illinois governors: Otto Kerner, Richard Ogilvie and Dan Walker.

In 1966, to provide for new benefits of the project, including water supply for Decatur, recreation and tourism, and pollution control, the Corps more than doubled the scope of the project. The original reservoir was to have been 8 miles long; this new lake would be 24 miles long. However, it would also extend the reservoir to just south of Monticello, ending agricultural production on 35,000 acres of land and displacing 110 families.

This expanded project would “reduce pollution” by allowing “low flow augmentation” to maintain a 2 foot depth of flow in the Sangamon River downstream from Lake Decatur. This would reduce pollution, not by actually reducing it, but by diluting the partially treated wastes from the Decatur Sanitary District just below the Decatur dam. Sometimes during those pesky dry periods, the flow of the Sangamon below the Lake consisted entirely of this partially treated effluent from this sewage treatment plant. This was “pollution control”, late 60’s style.

And upstream of the dam, one consequence of this “low flow augmentation” would be the expansive mudflats which would be exposed when the low flow water is released exposing the reservoir bottom. One report suggested these “malodorous mudflats” would “hamper recreation.”  As for me, it’s hard to imagine any fun recreational activities in malodorous mudflats, but no matter.

A 1973 report by the Illinois Environmental Protection Agency stated that this expanded Oakley Reservoir was expected to be “shallow, silty, turbid, algae ridden and frequently in violation of public health standards.”

I’m guessing these guys probably weren’t going to be hired to draft the tourist brochures.

Finally, in backing up the river 24 miles, this expansion would also necessitate permanently flooding Allerton Park with 5 to 6 feet of water. The Corps sought special “flowage easements” on 1100 acres of Allerton, including all of the bottomland forest. For the record, Allerton Park is about 1500 acres.

By this point, dear readers, you’re probably left wondering, well, why wouldn’t Decatur want a water supply that would frequently violate public health standards and create a nifty shallow, algae ridden reservoir that would inundate most of Allerton Park and would provide fun recreational activities in malodorous mudflats?

It turns out that this was precisely what these several Senators, Congressmen, the Corps of Engineers, and a parade of Illinois Governors and Decatur public officials did want.  Plans were set to begin construction in 1969!

But in threatening Allerton Park, the dam builders were about to find out that they had just messed with the wrong people.

…to be continued…

Appeared as "Notes from the River", Mahomet Citizen, February 21, 2012, by Scott P. Hays

Comments are closed.