Usually hillshades from a DEM are visualized from the northwest. Here I tried it both from 315 degrees (NW) and due south, 180 degrees. The results are very different. The topography of the lake appears inverted with the azimuth of 180. That is, the reservoir appears above the landscape.
Here is the contour map with a contour interval of 20 feet and an index contour every 100 feet. In ArcMap I made two separate contours.