Create Spline from code

GameObject gameObject = new GameObject("River");

SplineContainer splineContainer = gameObject.AddComponent<SplineContainer>();
splineContainer.Splines = null;

//Create a basic spline with a wiggly shape
int knots = 5;
float amplitude = 2f;
float length = 50f;
Spline spline = new Spline(knots, false);

for (int i = 0; i <= knots; i++)
{
    float t = (float)i / (float)knots;

    BezierKnot knot = new BezierKnot();

    knot.Position = new Vector3(Mathf.Sin(t * knots * 2f) * amplitude, 0f, (t * length) - (length * 0.5f));

    spline.Add(knot, TangentMode.Linear);
}

//Automatically recalculate tangents
spline.SetTangentMode(new SplineRange(0, spline.Count), TangentMode.AutoSmooth);

splineContainer.AddSpline(spline);