I'm studying the drainage of a lake and I have a DEM of the lake and the surrounding area. I already identified the shorelines of ancient lake levels at higher elevations, but I would like to keep draining the lake digitally to predict future drainage patterns. The DEM shows the even lake surface, although I have a bathymetry map which shows the depth of the lake bottom.

What can I do to lower the DEM surface to include the bathymetric information? Is there a way to overlay the bathymetry map and create polygons for the various depth contours, then use those polygons to assign new elevations to the areas within them on the DEM?

I have access to both qgis and ArcGis.

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    You could create a mask of your lake surface, then do some raster math (arcgis) to replace cells values in your dem where the mask is present with cell values in your bathy data. This is assuming that resolutions are equal. – Barbarossa Jan 9 at 16:37
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    What format is your bathymetry data in? – AWGIS Jan 9 at 16:50

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