This paper addresses the design of distributed systems with synchronous dataflow languages. As modular design entails handling both architecture and functional modularity, we propose a language-oriented solution, involving the extension of a synchronous dataflow language with primitives for program distribution. These primitives allow the programmer to describe the architecture of the system and to express where streams and expressions are located in this architecture. A distributed semantics is first proposed as formalization of the distributed execution of programs. A type and effect system is then provided in order to infer the localization of non-annotated values by means of type inference and to ensure, at compilation time, the consistency of the distribution. A type-directed projection operation allows us to obtain automatically, from a centralized typed program, the local program to be executed by each computing resource. The type system as well as the automatic distribution mechanism has been implemented in the Lucid Synchrone compiler.