Arakawa Fourth Groundsill

What the Arakawa Fourth Groundsill Is

The Arakawa Fourth Groundsill (荒川第四床固工, Arakawa Dai-yon Tokogatamekō) is a gravity-type concrete groundsill built in 1954 across the middle reaches of the Arakawa River at Kaminagura in Fukushima City, Fukushima Prefecture, and registered as a Registered Tangible Cultural Property (structure) of Japan on July 8, 2008. It is 184 meters long and 2.7 meters high, faced with Tanizumi stonework.

It is the only groundsill among the 15 registered Arakawa River Sabo Dams and also the farthest downstream, below Jizohara Dam at the head of the alluvial fan. It was registered in the second round, under number 07-0102, with official notice on July 23, 2008.

A Groundsill, Not a Dam

A groundsill is a low structure built across a river to fix the level of its bed. The Agency for Cultural Affairs states that the Arakawa Fourth Groundsill has no sediment-storage function; its job is to hold the riverbed in place and moderate its gradient.

The sabo dams upstream stand around 10 meters tall and catch sediment behind them. This structure rises only 2.7 meters. Its purpose is to stop the current from cutting the bed ever deeper and to break the slope of the river into gentler steps. What it lacks in height it makes up in length: at 184 meters it is the longest structure in the group, longer even than the 150-meter Arakawa Fifth Dam.

Groundsills remain part of the river's defenses today. According to MLIT's Fukushima River and National Highway Office, the No. 1 groundsill at the Arakawa sediment basin, built to stabilize the channel there, has nine culverts each with a radius of 3 meters.

The Same Corner Stones as the Fifth Dam

The Agency notes that on the Arakawa Fourth Groundsill the corner stones along the downstream edge of the crest are laid at an angle to the current so the masonry will not be torn out, a detail it calls unusual.

The same feature is recorded for the Arakawa Fifth Dam upstream. Both were completed in 1954. One is a 12-meter dam and the other a 2.7-meter groundsill, yet they guard the edge where overflowing water strikes hardest in the same way. Read side by side, the two descriptions show one approach to that corner applied in the same year.

The tanizumi facing sets each stone on a slant so that neighbors lock against one another, and it covers the whole of this long, low structure.

Seeing the Arakawa Fourth Groundsill

The Arakawa Fourth Groundsill is a working river structure. Viewing is free from the surrounding ground; you cannot stand on it or go down to the riverbed.

Look for the low step running 184 meters across the river and the slanted corner stones along its downstream edge. Both are easier to see when the water is low. The groundsill is well away from the dams clustered around Tsuchiyu Onsen, at Kaminagura, downstream of the Arai district where the Arakawa Archive Room stands.

Neither the Agency's database nor Fukushima City's list gives an owner, and no visitor information or parking has been published. Avoid entering nearby farmland or private roads, and stay off the riverbed after rain. Directions to the Archive Room and the dams are on the Arakawa River Sabo Dams page.