The simplest HEC-EFM applications involve one flow regime and one relationship as would be the case when modeling the effects of one scenario on one aspect of the ecosystem. Applications become more complex as the numbers of flow regimes and relationships increase. HEC-EFM models that investigate many alternatives for only a few species are likely to have a higher number of flow regimes and a low number of relationships. Conversely, modeling done to better understand the status of whole ecosystems will tend to have a low number of flow regimes and a higher number of relationships. As spatial scales increase from a single restoration site to long reaches in dendritic river systems, HEC-EFM applications often have location-based flow regimes that serve as multiple diagnostic locations for a single management alternative. The number of relationships may grow as relationships are duplicated and modified to test model sensitivities. HEC-EFM has several features to help support this spectrum of applications.