Following Cad lines

Hi I’m using Sketch up 2017 free edition and have imported some Cad lines with the help of an older version.
My intention was to use the Sandbox Contour tool to get a river valley shape. But the polygons go haywire.

Is there any way to turn up the detail of the polygon mesh so that it is more likely to follow my guidlines?

You could use Push Pull Tool, and then Move or Scale Tool for the bottom. Or Add detail from Sandbox Tools.
Or use Artisan.

With free plugins&tools, depending on what you want to do with that terrain, you could also use two free plugins (Loop Subdivision Smooth and Eneroth Fractal Terrain Eroder), which in combination with the Move tool and Sanbox tool to allow you to create that riverbed.

The Sandbox from Contours tool works incorrectly if the terrain contains vertical surfaces or ones with a negative incline. From two points that coincide in the XY plane it picks only the topmost one.