River flow regime is important role in determining the river morphology, water use and biotic composition. In Japan, normal discharge is determined after comprehensive condition of factors such as navigation, fisheries, landscape, maintenance of cleanness of river water, conditions of plant and animal habitats, prevention of salt damage and river mouth clogging, protection of river administration facilities and maintenance of ground water level. However, variation of river discharge is not included in Japanese river water management, because the knowledge of potential hydrological regime or relationship between flow regime and ecological system is lacking.

To reveal flow regime characteristic of river in Kyushu island, we propose 11 flow regime indicators which could explain seasonal change of stream flow at 52 watersheds. And we investigate the relationship between flow regime indicators and factors that affect the hydrology (physiographic factors, geological factors and vegetation factors).

As a result, indicator of rainy season (summer) was a function of rainfall amount, but, the correlation between indicator of dry season (winter) and rainfall amount was low. This result suggested that the amount of river discharge in dry season was influenced by physiographic factors, geological factors and vegetation factors than rainfall amount. Result of multiple regression analysis, flow regime indicators were well explained by the factors of physiography, geology and vegetation. The results indicated that geology of volcanic ejecta tends to stabilize the flow fluctuation, and that urbanization and plantation tend to increase frequency of disturbance and drought.