During four seasonal surveys in 1997--1998 the shelf circulation is dominated by a semipermanent cyclone originating in the higher-salinity waters from lower Aegean latitudes. Another cyclone in the Sporades Basin has the same origin and has a semipermanent nature in the upper ~200 m, while it is robust in all deeper layers. The variability in the upper ~200 m depends strongly on the local river discharge and on the inflow of Black Sea Water (BSW) (evident in summer and autumn). Consequently, the shelf cyclone coexists with anticyclones in the vicinity of the river mouths (February 1998 and May 1997) or coexists with an anticyclone due to BSW (September 1997). In July 1997 the upper ~20 m of the entire shelf is occupied by an anticyclone of BSW. The deep part of the Sporades Basin cyclone has possibly originated by deep water formation and spreading in the central north Aegean in the early 90s. In the numerical experiments the northern rivers form one plume that tends to occupy the northernmost part of the shelf due to the smaller domain scales therein. The western river (Pinios) plume expands north of the discharge site due to the abrupt shelf slope near the river mouth. Northerly winds can merge the northern and western river plumes, as in February 1998, while southerly winds can restrict the southward coastal current, thus separating northern and western river plumes, as in May 1997. A northward momentum imposed by an external Aegean flow along the east coast influences the spreading of north and west plumes.