On the Anomalous Phenomenon Caused by the Dry Spell in the Early Summer of 1958 in the Tidal Estuary of the River Tone

Abstract

In the year, during the period from spring to early summer remarkable rise of salinity in the estuary water of the River Tone and the corresponding change of the aquatic livings were observed. The results of studies conducted by the present author are as follows:
The trace of the oceanic water was detected at the station upstream about 45 km distant from the mouth of River Tone even at the time of low tide. At the station upstream about 27 km distant from the river mouth, the ratio of oceanic water and fresh water indicates about 50% to 50%, where marine organisms (such as mackerel, octopus) were found against normal fresh water organisms (eel, silver-carp, corbicular) which died or escaped away.
Such anomalous condition in this district has been aroused by the extremely dry weather which continued from spring to early summer in that year. It resulted in the decrease of fresh river water and as its consequence the inflow of oceanic water into the river (especially in the lower layer) increased. In addition to it, some other factors to strengthen the upstream flushing of sea water into the interior of the river might be considered, i.e. the deepening of the river floor by the artificial dredge on river bottom under the flood prevention works or by the rise of estuary water level effected by the occasional storms passing by the river.