Solutions to spatial environmental problems often require the integration of dynamic simulation models within GIS to create spatial decision support systems (SDSS) that can generate responses to theoretical ”What if?” scenarios. Extending this paradigm to a collaborative spatial decision support system, however, faces significant challenges. This includes the inability of computationally intensive models to provide real-time results, and the inability of novice end users to effectively parameterize the models. Effective solutions to these problems proposed here include the use of ”cause-effect” models to link inputs to outputs for a limited number of scenarios, as well as utilizing software agents that assist novice users in determining the correct input parameters for the models. Examples from the St-Esprit watershed SDSS serve to elucidate the proposed solutions.