Stream Processing Applications analyze large volumes of streaming data in real-time. These applications, consist of data sources, which produce raw streams, and processing elements, which operate on the streams to produce new derived streams. In this paper, we describe an OWL-based model for describing these stream processing components. The streams produced by data sources are described as RDF graphs, consisting of OWL ABox assertions. The input requirements and output capabilities of PEs are described using RDF graph patterns. The main use of this model is to allow automatic composition of applications that produce certain desired results. Our model allows us to answer two key questions in automatic composition: can a certain stream be given as input to a processing element, and what are the new streams produced as a result. Based on this model, we outline a planning approach to automatic application composition.