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.
Release notice
Release synchronization in progress: the Creator Store text and media currently describe the validated local v0.1.2 candidate, while the buyer binary is still documented as v0.1.1 until the exact v0.1.2 package is uploaded, moderated and verified publicly. Purchase links are paused here so the listing cannot be mistaken for proof that the new binary is already live.
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
The number on the panel is measured AFTER the write, read back out of the terrain rather than carried over from the plan. On the live v0.1.1 acceptance course: 52 of 52 stations held water on a 404-stud river, worst rise 0.000.
What it does not do
It refuses what it cannot do. An uphill course is rejected with the number, a trench past the depth limit is rejected, and a column with a cave in it is left alone rather than filled solid. One Ctrl+Z takes the whole river back.
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; 1,690 voxels changed; rise 0.000; Fall error 0.005 at 216 studs along course. Undo/Redo and repeat-cut refusal passed. Gates: 43/43 Studio tests; 91/91 checks.
Editor-only; 4-stud voxels. No runtime scripts, HTTP, telemetry, currents, erosion, waterfalls or lake merging.