Watercourse demarcation studies require detailed hydrologic and hydraulic computations. Demarcation studies of watercourses in Greece are carried out according to Law 3010/2002. The studies require extensive review of environmental and hydraulic aspects of the existing and projected conditions of the watercourse and its watershed. Advanced computational capabilities should be used for better determination of the parameters involved, both for simple methods as the rational method and more detailed ones as the application of synthetic hydrographs. The demarcation lines have to surround the flooding area, the existing banks and any technical works associated with the watercourse, such as paths or maintenance roads of regulation works. The use of digital elevation models and GIS facilitate determination of morphologic characteristics and distributed parameter values for estimation of design discharges for demarcation studies of watercourses. Free software like HEC-HMS or HEC-RAS is particularly useful in demarcation studies of water courses and for the determination of flooding.