Program federation is assembling a software system from cooperating but independent application programs. We present DeCo, a declarative approach to creating and coordinating federations, and discuss its application in the domain of scientific computing. DeCo is meant to automate several aspects of the typically manual process of program federation. A federation is expressed in the functional language Haskell, extended with constructs for describing and coordinating the participating programs and data files. The declarative expression of a federation in terms of data flow among the component programs captures synchronization requirements implicitly and exploits the inherent concurrency automatically. Haskell compilation, notably its rigorous type checking, ensures the consistency of the federation. Operation of the coordination framework is demonstrated on a federation of Fortran programs that simulate environmental processes in the Neuse River estuary of North Carolina.