Hello!
You can request free evaluation license of ArcGIS Spatial Analyst if necessary. However, you can also replace the values of the elevation surface by mosaicking a constant raster with existing surface.
Here's what could be done.
- If you know constant value and the area(lake boundary for example) you can create a raster with constant elevation value by converting polygon to raster. You can use original elevation raster for cell size and snapping.
- Once you have the constant raster created, you can mosaic the two together. Make sure you select correct parameter for pixel type, number of bands, and mosaic operator.
Hope this helps.
Thank you,
Prasanta.