I have a water surface elevation raster that was generated in Geo-RAS from an existing HEC-RAS model. I am interested in subtracting this raster from an elevation raster that contains LiDAR data to perform some overbanking potential analysis in conjunction with some river restoration work. The challenge I am facing is that the water surface raster has gaps in coverage at some of the river bends where LiDAR data exists, so my resulting analysis does not cover these banks. I would like to simply "stretch" the water surface raster to cover the data gaps and it is okay if the values in the cell are simply carried through from the ones at the border of the original raster. Is this possible?