This code is now being maintained in Bitbucket, here
ORD Stormwater Management Model (aka "SWMM")
This is the official SWMM source code repository maintained by US EPA ORD, NRMRL, Water Supply and Water Resources Division located in Cincinnati, Ohio.
SWMM is a dynamic hydrology-hydraulic water quality simulation model. It is used for single event or long-term (continuous) simulation of runoff quantity and quality from primarily urban areas. SWMM source code is written in the C Programming Language and released in the Public Domain.
The source code distributed here is identical to the code found at the official SWMM Website.