Lower Eighteenmile Creek

The ground

Old clay hills, still mostly green

Lower Eighteenmile sits in the Piedmont foothills: weathered red-clay ridges of the Cecil–Pacolet family, tilting south toward Lake Hartwell. Nearly nine in ten acres share a single soil behavior — hydrologic group B — so the whole watershed drinks water at about the same rate. That makes it unusually easy to read.

Most of it is still green. In 2023, forest and farmland together cover about 60% of the watershed. That is the good news, and the opportunity — there is a lot left to keep. It is also changing faster than a drive through it would suggest.

What's changing

The ground is hardening

Impervious cover — roofs, roads, parking, anything the rain runs off instead of soaking into — has climbed from 4.7% of the watershed in 1985 to 8.4% in 2023. In plain acres, that is about 580 new acres that now shed water like a roof: a 77% jump in a single generation.

Spread across 25 square miles, 8% still sounds modest. The point isn't the number today — it's the slope of the line, and what even a few points of hardening do to the water once it lands. Below, the same watershed seen as land cover and as impervious surface.

Where the green is

Forest traded for developed ground

Here is the whole land cover, year by year. Drag the slider to move through time; click a wedge to trace one class. Since 1985 the watershed has lost about 1,320 acres of forest and gained about 1,540 acres of developed land — very nearly an acre-for-acre trade.

Donuts compare composition, so a 25-square-mile watershed and a whole region sit side by side fairly. Read across places, not sizes: Lower Eighteenmile is still a green watershed hardening at its edges, where the campus is already mostly built and the Upstate and state carry big reservoirs in their "open water" share.

What the water does now

Same storms, more runoff

Hardening changes how the watershed handles rain, and we can put a number on it. Using the standard curve-number method — the same TR-55 approach an engineer uses to size a culvert — the watershed's runoff curve number rose from 64.4 to 65.7 as it hardened. A point and a third. Here is what that does to real design storms:

What we can do

Keep the water where it lands

The fix isn't exotic. Every acre we harden exports a little more responsibility downstream. The response is to stop exporting it — to keep the water at elevation and let it sink in, across thousands of small distributed places instead of a few big pipes.

Rain gardens, restored wetlands, regraded swales, forested buffers, farm ponds, better-graded new development — a watershed's worth of small moves that, added together, hand the creek back the behavior it had when 90% of this ground was still soaking up its own rain. None of it is speculative; all of it is buildable now.

That is a design problem, and a solvable one. The atlas exists to make it legible — to show where the water goes, where it is changing, and where the highest-leverage places to slow it down actually are. Lower Eighteenmile is watershed No. 01. The method is the same for the next one.

The receipts

Every number, from open data

All figures on this page are computed locally and reproducibly from public sources — no proprietary data, no black boxes. The runoff model was validated out-of-sample: calibrated on the full-watershed StreamStats point, it reproduced a held-out design point within 0.5% of its independent value.

Surface-temperature (heat) layer from Landsat is in progress and will be added as the atlas grows. Impairment status: SCDES 303(d) — Lower Eighteenmile Creek listed for turbidity and total nutrients, no TMDL on record.