The Village That Chose a Riverbank on Purpose
Mahomet was platted in 1832 on the west bank of the Sangamon River specifically because settlers wanted the water -- making it the first community established in what is now Champaign County. That deliberate river-adjacent siting is still the defining fact for any basement dug near Mahomet's original townsite.
Basements Near a River Headwater
A basement near the Sangamon's headwaters deals with a seasonally rising water table in addition to the clay-and-drainage issues common across the county, which is a different waterproofing problem than a basement built on open, un-watered prairie. Exterior drainage planning in Mahomet should start with proximity to the river, not just soil type.