River Mouth Fluctuation of Small River in Iwate

The salmon is the main marine product of Iwate prefecture and salmon culture projects are very active: therefore it is very important to prevent the mouth of a salmon culture river from being blocked by sand deposit. However civil engineers had not been concerned with fluctuations of such small rivers. In this study, field observations were carried out at 41 river mouths in Iwate to comprehend the situations of small rivers. Observations show that at 75% of river mouths both water depth and river width are not enough for salmon returning, and that the time scale of river mouth fluctuations depends on topographic features of coast line near the river mouth.