ABSTRACT
ABSTRACT: The Toyohira-gawa alluvial fan is an urbanized fan. A billion tonnes of river runoff flows every year on the surface of the fan, and is the source of tap water to the 1.9 million people in Sapporo region. Moreover, there are also 300 million tonnes of groundwater stored in the fan. The river Toyohira-gawa is recharged by both rain and snow, in approximately equal amounts. In addition the fan is recharged from the river water. Recharge from the riverbed to the underground is estimated as 321mm/y, while 277mm/y is from the land surface. Though the volume of discharge from the groundwater to the river cannot be estimated, it is apparent that there is an active interaction between surface water and groundwater. This is evident from several losing and gaining water segments on the riverbed.
