HEC-1 is software that was developed by the  US Army Corps of Engineers to estimate river flows as a result of rainfall. It was written in the FORTRAN language and until 1984 could only be run on a mainframe computer. When desktop computers became popular the program was ported to the PC. Engineers and scientists still use it today because of its ability to model a wide range of natural hydrologic systems.
| http://www.hec.usace.army.mil/software/legacysoftware/legacysoftware.html Army Hydrologic Engineering Center