The Watershed Atlas  ·  No. 01 — Lower Eighteenmile  ·  the atlas  ·  No. 06 — California Branch

Coastal  ·  Soggy — the full sponge

The Watershed Atlas · No. 06

California Branch

The outlier — a low, flat, wet Coastal-Plain stream near Moncks Corner, part of a stream-buyout and restoration effort. It’s in the atlas on purpose: it reads nothing like the Piedmont creeks, and the contrast is the lesson. Not every watershed sheds water the same way.

4.4 sq miMoncks Corner, Coastal Plain86% poorly-drained soil (C+D)low, flat & wet
Terrain and stream map of California Branch
Terrain & streams · USGS 3DEP LiDAR hillshade + NHD flowlines. Note how little relief there is compared with the Piedmont watersheds.

The ground

A different kind of ground

California Branch drains about 4.4 square miles of the Lowcountry Coastal Plain — flat, sandy-to-clayey, and wet. Where the Piedmont creeks sit on well-drained group-B clay, this watershed is 86% hydrologic group C and D: soils that hold water near the surface, with wetlands and ponded floodplain doing much of the storage.

That single fact changes how you read everything else.

58%
Soils in hydrologic group D — poorly drained
13%
Impervious surface, 2023 — just past the 10% line
50%
Developed land, 2023 — up from 44%

What's changing

Past the line, but read it carefully

Impervious cover rose from about 11% to 13% — just over the 10% degradation threshold. But on Coastal-Plain soils that already run off readily, hardening does less additional work than it would in the Piedmont: the ground was never a great sponge to begin with.

Impervious cover, 1985–2023

Share of the 2,846-acre watershed that is impervious. Dashed line = the 10% degradation threshold. Source: USGS EROS Annual NLCD, 30 m.
Land cover map
Land cover, 2023 · NLCD.
Impervious surface map
Impervious surface, 2023 · NLCD.

Where the green is

Forest to development, on wet ground

Developed land went from about 44% to 50% of the watershed since 1985; forest fell from 14% to 9%. The composition is more mixed than the Piedmont creeks, with the wetland and low-ground share that comes with Coastal-Plain terrain.

California Branch

Land cover by area · 2023

Lower Eighteenmile · No. 01 (Piedmont)

Composition, not size · same year
2023

Water quality — the 303(d) list

What it drains into

California Branch is small enough that it isn't a separately monitored segment on the state's 303(d) list.

But it drains straight into the low Coastal-Plain waters of the Cooper River / Lake Moultrie system — which is listed, for the sediment and low-oxygen pressures typical of altered Coastal-Plain streams. Whatever this watershed sheds, it sheds into an already-impaired water.

Source: EPA ATTAINS (the national clearinghouse for state Clean Water Act §303(d)/305(b) assessments), current reporting cycle. “Cause” = the pollutant; “use” = what it keeps the water from supporting (swimming, aquatic life).

What the water does now

Flashy for a different reason

Here the runoff numbers come with an asterisk. On these poorly-drained soils the natural curve number is already high (~78), so the model shows a modest hardening effect — but the standard TR-55 method assumes soils and slopes the Coastal Plain doesn’t have. Read the direction, not the decimals:

Runoff volume per storm — 1985 vs 2023

Runoff by storm size, curve-number model. ⚠ Coastal-Plain caveat: wetland and floodplain storage violate TR-55’s assumptions, so these are directional screening numbers, not design values. Storm depths from NOAA Atlas 14.
The soils, not the pavement, are the story here.

California Branch runs off readily because it’s flat and wet, not mainly because it’s hardening. That’s the contrast worth holding onto: the same 13% impervious means something very different here than it would on a Piedmont hillside. Stewardship on the Coastal Plain is about wetlands and floodplain connection, not curve numbers.

Why it’s in the atlas

The exception that frames the rule

Put next to the Piedmont creeks, California Branch makes the pattern legible: hardening matters most where the ground used to soak water up. On the Coastal Plain, where it never did as well, the levers are different — protect the wetlands and the floodplain that do the storing, rather than chase a curve number.

Knowing which watershed you’re in is the first act of stewardship. This one proves why.

Slow it · spread it · sink it

The receipts

Every number, from open data

Delineated fresh from its Coastal-Plain outlet via USGS SS-Delineate (4.44 sq mi). Every figure computed locally from public sources; the runoff model carries a Coastal-Plain caveat.

Watershed boundaryUSGS SS-Delineate v1 from the Coastal-Plain outlet near Moncks Corner — 4.44 sq mi.
Impervious & land coverUSGS EROS Annual NLCD Collection 1 — Fractional Impervious Surface and Land Cover, 30 m, 1985–2023.
Streams & terrainUSGS NHD flowlines; USGS 3DEP 1 m LiDAR hillshade.
Soils / hydrologic groupUSDA SSURGO via Soil Data Access — area-weighted.
Runoff modelTR-55 / SCS Curve Number (Coastal-Plain caveat: directional screening only).
ImpairmentsEPA ATTAINS §303(d)/305(b) assessment service, current cycle.