Center-line points as [x, surfaceY, z], ordered source → mouth (the
flow direction is point i → point i+1).
Full strip width (m). Rivers want the WET channel width, not the gorge width.
OptionalfeatherEdge feather (m). Keep it well inside the banks — a narrow river's wet channel is only a few meters wide, so a wide fade sinks the water exactly where it should show. Default 30% of the half width.
OptionalstepTarget subdivided link length (m). Default 6.
OptionalflowDerive flow segments along the spline (surface-detail advection).
true for defaults, or an options object.
One authored water spline: a center-line of
[x, surfaceY, z]points (surfaceY = ABSOLUTE world height of the water surface there — what a level designer reads off the terrain), plus the strip shape.