Problems of the Nagara-River-Dam at the mouth


started since July 6, 1995


show a map of Nagara River


1. Destruction of sea mingled with fresh water area (down stream of the dam )

A. The high salt density of the sole layer (the salt density incline of perpendicularity direction fixes it)
B. Low oxygen of the sole layer (a perpendicular stratum formation interfere with diffusion of oxgen to bottom)
C. A countercurrent of the sole layer (a countercurrent occurs by the bottom layer because the seawater outer layer is rolled up in fresh water, and it is carried away, and to supplement this)

On this account, black sludge it lies on bed of river from 3 km than the river mouth to a dam (5.4 km than the river mouth) , and Corbicula japonica doesn't survive almost. It was looked at a large quantity of corpse of Corbicula japonica and the slightly numerical lugworm class which survived (July 1966). However, nothing remain already in sludge except for trash (July 1997).

upper left: sludge of the bottom of the river (5 km, May 1996)
  upper right: remove sludge with mesh corpses of corbiculas (July 1996)


  lower left: sludge of the bottom of the river (5 km, July 1997)
  lower right: remove sludge with mesh trash only (July 1997)


sand of river bed of Ibi River with live corbiculas (5 km, July 1997)

We call this "the river mouth bottom of death".

show an illustration

We can confirm the accumulation of sludge of maximum 1.1 m (a calculation value) with ultrasonic sound prospect.

show the results of ultrasonic


2. Changing in lakes and marshes (the upper reaches of a dam)

A. No oxygen state of the bottom of a lake in summer
B. Abnormal outbreak of alga (chlorophyll a values of 80 - 150 ug/l were recorded. It usually was several ug/l in the stream.) Moreover, it has been reported outbreak of green algae annually since 1995 when the dam started.
C. Increase of midges (chironomus midges hardly inhabited at 6-10 km than river mouth for salt water going up. Outbreak of midges of being in a different class begins currently.)A cause of allergic asthma that midges is next to mite.

D. Sedimentation of sludge. Sluge is sdimenting on the bottom of upper side of the damm,too. Many bubbles of methane were seen during autumn and spring

sluge on the bottomn of upper side near the dam; May 1998
E. Regression of the reed-field. Reed-fields are greatly stolen because of sinking of stalks.

View from 6.9km point on August 30, 1996 (photo by Prof. Yamauchi)

View from the same point on September 2, 1988

3. Obstruction of going up of wandering fishes

A. An interference with descent of sweetfishes immediately after birth to sea
The Ministry of Construction reported that sweetfishes which was below 42 km spot in autumn, 95 was 250 millions. It is in comparison with or a result with a few in a quantity of descent of an average year. By our investigation, it was identified that 80-90 % of very young fishes were lifeless immediately at the top of the dam. There is not bait to the upper reaches of a dam. Very young sweetfishes must swim by own strength, because a flow of a river is loose, moreover, sweetfishes can't step on the tide. Data by The Ministry of Construction with showed the annihilation appear if doesn't fall by the eighth day.
B. A poor catch of sweetfishes and Oncorhynchus masou
C.Crushing influence of Japanese common eel fishing

4. Influences to Ibi River

Ibi River and Nagara is next to each other by a dike.The branching bay of the latter was stopped in a dam, but the former is not. On this account, the tide gets into Ibi River like a brick.

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