I'm working with ArcGIS 10.7 to investigate lakes in the mountains. I have created a 2km "hazard buffer" around every lake-TIN showing instable slopes (>40°) above the lake surface from where a rockfall could hit the lake (see example below).

However, not all of the slopes inside the buffer are pointing towards my lake, some of them are on the other side of ridges, naturally.

I need to identify every mountainside with a slope >40° that is pointing towards my lake and classify them depending on their distance and height difference to the lake. The distance and height difference should not be too difficult to figure out (I hope) - but I struggle with the slope detection.

How could I use aspect or some other criteria to determine which slopes to keep?

EDIT: I know that my question is relatively broad. However, I'm not looking for a detailed workflow but for hints or tips on what I might look into.

  • 1
    Have you considered treating rocks like water? Seems like the FlowDirection tool might be useful. Feb 6, 2020 at 15:40

1 Answer 1


First generate a flow direction tool like Kirk suggests. Then you can use your lake polygons and the watershed tool to determine those contributing area for the lakes. Use the resulting watershed output to mask your slope/height raster to ensure you are only considering areas where rocks can actually fall into the lakes.

  • This will include rocks falling into streams almost 2 km upstream from lake.
    – FelixIP
    Feb 6, 2020 at 18:49
  • You are right, of course. I used the 2km merely as a starting point, I will have to look into the literature for a more appropriate buffer size. Feb 7, 2020 at 8:19

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