The Second Question
What Is Under It — Clay, Sand Seams And Test Pits
The watershed decides whether a site can fill. The dirt decides whether it can hold, and whether you can afford to build it. These are two separate questions and the second one is where lake projects go wrong quietly, because the surface tells you almost nothing about it.
Central Illinois is generous here, up to a point. The heavy Illinoian till under most of Coles, Cumberland, Douglas and Moultrie counties is close to ideal impoundment material: dense enough to seal a basin on its own, plastic enough to knit into a compacted embankment, and usually available in quantity on the same property the lake is going on. That last part matters more at lake scale than anything else on this page — a project that can cut its embankment material out of its own basin is a fundamentally different job from one trucking select fill in from somewhere else.
Sand seams and gravel lenses are the thing to find first
Glacial ground is layered, not uniform, and it was laid down by water as well as ice. So inside a single field you can have tight till at the surface and a sand seam or a gravel lens two or three feet down — a permeable layer, sometimes only inches thick, running through ground that looks entirely trustworthy from on top. Under a basin, that layer is a drain. Under a dam, it is worse: it is a continuous path beneath the structure that no amount of compaction in the fill above can close.
The risk goes up exactly where lake sites are attractive. The draws that make good impoundments are draws because water has worked them, and sandier river-influenced material follows the drainage — along the Embarras, along the Kaskaskia, and in the bottoms below them. A good draw and a bad seam frequently occupy the same ground.
Test pits, before a stake goes in
There is one honest way to know, and it is not a soil map. We dig test pits — on the dam centerline first, then across the basin footprint, and deeper than the deepest cut the design calls for, because a layer below the excavation still matters under a loaded embankment. What we are reading out of them is straightforward: how deep the topsoil and root zone go, whether there is tight material to key a core trench into and at what elevation, whether anything permeable runs through the profile, and how much usable clay the site actually owns.
An excavator and a couple of hours buys all of that. Finding a seam after the basin is open costs a redesign, and finding it after the pool is up costs the lake. It is also how the question gets answered honestly rather than optimistically: on some sites the pits say move the dam upstream, and on a few they say this is the wrong spot for an impoundment. We would rather say that at the walk.
Embankment lake or dug basin — the ground picks
Nearly every impoundment is one of two shapes, and which one a site gets is decided by relief, not preference. An embankment lake dams a draw: the valley provides most of the volume, the watershed is already aimed at the pool, and the earthwork concentrates into the dam and the spillway. It is far and away the better value per acre-foot of water where the ground offers it — the rolling country toward the Embarras and the Little Wabash draws, or south and west of Mattoon. A dug basin is what flat ground leaves you: every yard is excavated and then has to go somewhere, spoil handling becomes the largest single item on the job, and there is no valley doing free work. On the flat prairie north of us that is simply the job.
How deep, how long, and what season
Three questions come up on every call, and they all come back to the same thing: a lake is an earth volume problem, not an area problem. Depth is the lever people underestimate. Most farm and recreation ponds around here sit at eight to twelve feet of average depth, and anything meant to hold fish through a central Illinois August wants fifteen feet or more somewhere in it for a thermal refuge. Going deeper adds volume across the whole basin rather than just the middle, so depth moves the earthwork — and the price — further than surface acres do.
Duration scales with that volume, not with the acreage. An acre of water at twelve feet on the deep end is roughly eight to nine thousand cubic yards and two weeks or more of dirt work. Past about two acres you are into twenty thousand yards and up, a month or more on site, and a dam where sizing and compaction stop being details and become the entire schedule. Weather is inside those numbers, not alongside them: a lift that is too wet to compact stops the job until it is not.
Season is not a preference either. Late May through early October is the window, with June through September the heart of it, and the reasons are all mechanical. The ground is dry enough to cut, haul and compact — saturated clay slumps when you cut it and refuses to reach density, which is the one failure a dam cannot carry. The basin can be worked dry. There is still time for the pool to come up on summer runoff before the hard fall rains. And bank seeding laid in June or July establishes before the first frost, where late-fall seed washes off in the first big storm. Spring is for walking the site, digging the test pits and getting on the schedule — not for moving dirt. Work on an existing impoundment is the exception and can run later into the fall, because it does not depend on new bank vegetation the way a fresh dig does.
Either way, the water is surface runoff off your own ground and the pool is held by dense native material — the bottom and wetted face scarified, moisture-conditioned and recompacted as the basin is shaped, and the dam keyed and built in compacted lifts. That is the only mechanism we build on, at pond scale or lake scale, and it is why the soil investigation is not paperwork. It is the design.
