Formation and loss of pools at the middle reaches of the jinzu and shou rivers in recent years

摘要

Formation and disappearance of pools, which are very important for the survival of fishes, were investigated during the period from 1997 to 2000 at the middle reaches of the Jinzu and Shou Rivers. These 2 rivers are important as main fishing groundsof ayu, Plecoglossus altivelis, and masu salmon, Oncorkynchus masou, in northern Japan. In the Jinzu River the number of large pools deeper than 2 m decreased from eighteen in 1997 to eleven in 2000. Those of the Shou River which numbered five in 1997,decreased to three in 2000. The existence of pools in both rivers was highly unstable during the period; for instance 30 pools were lost and 21 pools were newly formed. The number of pools that remained unchanged during the period was only four in the Jinzu River and none in the Shou River. The pools in the Jinzu River tended to decrease in number. The classification of river-structure type Bb (representing a typical middle-reach type) which is characterized by a combination of riffles and pools could not be found in good condition in both rivers. It is considered that these changes in river character were mainly caused by river conservation works such as bank protections, and they have negatively affected ayu's and masu salmon's living habitats.