地図

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A city map showing the detailed topography of Sapporo at the time it was awarded city status was found in the Sapporo City Archives. We digitized the contour lines on the map and restored the topography of the Toyohira River alluvial fan at the time on a geographic information system. We also identified the locations of springs (“memu” in Ainu) in the area of the fan from the literature and obsolete maps, and considered the hydrological properties of the fan.

The results showed that the Hokkaido Government Office and Sapporo City Hall were built on a broad ridge, and the distribution of springs differed on the two sides of the ridge. The springs on the west side tended to be at higher altitudes, although some were at the bottom of the fan at low altitudes. The springs at high positions typically flowed beneath the surface to the end of the fan, as the surface water gathered in higher catchment areas, such as small valleys, and penetrated underground. In comparison, the springs at low altitudes originated from the abundant groundwater in the fan because there was no distinct catchment area upstream, and the springs flowed from deep within the gravel of the fan. There were relatively few springs on the east side of the ridge because of the small catchment area.

Former streams along the Toyohira River are considered to have flowed in the spindle-shaped riverbed typically found in a medium-sized fan from their distribution forms. The water level in the upstream of the Toyohira Bridge was easily to rise, because width of the river was reduced when the bridge was built in 1878. Therefore, the flooding of the Toyohira River is thought to have been caused by overflowing in the upstream and mainly flowing out in the former riverbed.

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