We investigated distribution and habitat conditions of Ephemera orientalis in the middle
and downstream regions of the Asahi River in Okayama Prefecture. We found 11 habitat sites of
this species. They co-existed with E. strigata upstream of Nakamaki. After two floods caused by typhoons
in September and October, 2017, several habitats (e.g. downstream of the Asahikawa Dam,
or on artificial crossing structures in river branches) were lost. Their habitats were in stagnant or
calmly-flowing water areas (including points higher than 120 m a.s.l.). Multiple regression analysis
between their habitat density and the environment conditions revealed a correlation between habitat
density and incidence of loose stones.
