It was mid May and there was a moderate hatch started after midday that got more intense in the evening hours. A low land stone bottom river, 20 to 30 m. wide, with water weeds growth.
Very tiny mayfly. Body lenght was about 4 mm and front wing 6 mm.
Unfortunately the only specimen I brought home had a damaged wing and could not turn to imago. A closer view with the microscope confirmed a hind wing with small costal process and two divergent veins. The hind wing was > 3 times long than wide.