Unfortunately right now landscape will not generate a distance field. Also any sort of spline mesh will not be able to generate a proper distance field.
It should be possible though to provide either separate geometry for the bottom (without splines), or use a height function or height map for the bottom part. You would need to compute it separately and combine it with the distance function result using Min or Multiply. You could also try to use that result to blend in a flowmap which could be as simple as a blurry normalmap generated from your height data. Sometime next week I may try to play with combining all this stuff.