For starters in providing a solution, can you provide the following:
1. Which software package are you using? ArcGIS Desktop? ArcGIS Pro? Another?
2. What version is the software?
3. If it is ArcGIS Desktop or ArcGIS Pro, do you have access to the Spatial Analyst or 3D Analyst Extensions?
4. Is vertical elevation data available for your planning area? What format? Elevation points, a raster surface?
5. Are the ridgelines already derived or will part of the task be to derive them to then use in the analysis?
Chris Donohue, GISP