I am west of you in South Central Illinois. Anthropic provides good questions. Based upon my lake and experience, I would encourage you to have a larger dedicated spillway than you might first consider too.

My lake is in the 17 - 21 acre range (depends if count sediment basins, etc.) and its watershed is one hundred and some acres (maybe 170?) of mostly farm fields and CRP. I used the emergency spillway as the main overflow for some time - until a major rain event occurred, it cut a trench big enough to bury a bulldozer, and made me fear major repair expenses. There was apparently some non-clay dirt in the undisturbed hillside over which my spillway ran. I now have an 8" siphon installed and drain it down some whenever it appears there will be a big rain event. I still want to put in at least one or two dedicated overflows a little lower than the emergency spillway just in case. Play it safe in advance so you don't risk a big rain event taking out your dam. That is a relatively huge watershed ratio for this general area so a much smaller rainfall will have an even bigger impact on your lake.

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