Runnel
Place a few points across your map. Runnel cuts the channel, snaps the bed to the voxel grid, tapers the banks and fills it with water.
What you get
It solves the surface as a falling curve before it writes anything: every station sits at or below the one before it, and never above the ground it runs through.
How it holds up
In the published v0.1.2 acceptance run, the result was read back after the write: 56 of 56 stations held water on a 414-stud river. Deepest cut: 16.00 studs; 1,689 voxels changed; worst rise: 0.000. This is one measured course, not a guarantee for every terrain.
What it does not do
Runnel refuses uphill routes, cuts beyond Max Cut, missing ground, existing water, caves/overhangs and clipped terrain before mutation. A successful cut is one Studio Undo step. No currents, erosion, waterfalls or lake merging.
The listing, word for word
BUILD A TERRAIN RIVER FROM A PATH.
Select Parts source-to-mouth. Preview the channel, then carve its bed and banks and add water.
MEASURE FIRST. Check length, stations, drop and deepest cut without editing. Tune Width, Depth, Bank, Fall and Max Cut.
VERIFY AFTER. A cut passes readback only if every station holds water and sampled steps meet minimum Fall within 0.01 stud. A successful cut is one Studio Undo step.
UNSAFE ROUTE? REFUSED. Before mutation, Runnel rejects a mouth above its source, cuts over Max Cut, missing ground, existing water, caves/overhangs, clipped terrain and oversized jobs.
Acceptance run: 414 studs; 56/56 wet; deepest cut 16.00 studs at 38 studs along course; 985 columns dug; 520 wet; 1,689 voxels changed; rise 0.000; Fall error 0.005 at 216 studs along course. Undo/Redo and repeat-cut refusal passed. Gates: 46/46 Studio tests; 95/95 checks.
Editor-only; 4-stud voxels. No runtime scripts, HTTP, telemetry, currents, erosion, waterfalls or lake merging.