Solved! Go to Solution.
The problem was due to the fact that the version of Arc Hydro was not correct. There was a change in ArcGIS between 10.1 and 10.2 with the name of the grid_code field generated by the Raster To Polygon tool. This field is used in Arc Hydro to dissolve the newly created polygon features So if you do not have the correct version of Arc Hydro for the version of ArcGIS you are using, you will see this error.
Christine Dartiguenave
Esri Water Resources Team