For the assessment of ecohydrological effects of dam construction in headwater regions an evaluation method is presented based on the socalled "ecological risk assessment". This method allows the determination of the spatially detailed ecological risk for various environmental factors based on the aggregation of two ordinal values: an evaluation of the present ecological state of the area under consideration and its forecast degree of damage. Here we focus on the integration of cause-effect relationships between abiotics and biotics and thus on interdisciplinary cooperation between the fields of civil engineering and ecology. The methodology is implemented in a complex software system. First a geographical information system is used for the management of the spatial database. A computer Expert System will be linked to the GIS, thus allowing an efficient implementation of the evaluation procedure. This way the decision process for finding the ecologically best project alternative is supported.