Terrain (or TIN) to Raster results in areas with erroneous data

[MollyJackson]

We regularly generate large DEM rasters by converting Terrains (or sometimes TINs) to 3' rasters. Large rivers in our terrains are controlled using waterlines as hard breaklines. They look great in the terrains. But when we convert the terrain to a raster, there are often large blocks of erroneous data in the water bodies. I'm assuming this is due to lack of terrain data in the middle of the river. Are there any suggestions on how to fix this issue? We are currently "patching" river rasters into the ground rasters by mosaicking them together, a tedious process. 

Thank you!