EFDC+

EFDC+ is DSI's actively developed, open-source continuation of the EPA's Environmental Fluid Dynamics Code: 3D hydrodynamics, salinity and temperature, sediment transport, toxics and full eutrophication kinetics for rivers, lakes, estuaries and coastal seas, with modern additions (sigma-zed vertical grids, OpenMP/MPI performance, hydrokinetic devices).

The engine is free and open on GitHub; DSI sells EFDC_Explorer, the GUI that most practitioners use to build and visualise models. That combination — open regulatory-lineage engine, paid productivity layer — makes EFDC+ the most accessible full 3D water-quality stack going.