As part of the Causal Analysis/Diagnosis Decision Information System (CADDIS), USEPA is developing a series of web-based, interactive conceptual models (ICMs) for priority stream stressors.  The aim of these ICMs is to provide CADDIS users with information that will strengthen causal assessments of stream impairment.  Given their cognitively supportive characteristics, interactive conceptual diagrams provide a promising way of organizing and accessing this information.  The foundation of each ICM is a simple graphic representation of the potential linkages between a single stressor (e.g., phosphorus, sediment), its common sources, and its likely biological effects.  This box-and-arrow diagram then serves as a both a visual tool and a template, which users can “query” (via hyperlinks) to access more detailed information.  For example, users can select specific shapes in the diagram, to retrieve information on references which support the hypothesized relationship between those shapes.  The first phase of the ICM for phosphorus is now available in external review form at the CADDIS website, and similar models currently are under development for additional stressors (e.g., nitrogen, sediment).  The next phase of ICM development will involve incorporation of additional layers of information, dealing with topics such as measurement techniques and stressor interactions.