2013 Volume 122 Issue 4 Pages 709-722
The upper reaches of the Azusa River in the Kamikochi Valley, a gravel-bed river with braided channels, are characterized by Salix arbutifolia occurring in patches and as isolated trees in the active riverbed. This study aims to clarify the relationships between geomorphological dynamics of the riverbed and environmental diversity of the active riverbed in the Kamikochi Valley in the establishment and growth of pioneer plants. The discussion on geomorphological processes is based on five geomorphological maps of the observation site in the Kamikochi Valley, which were made every summer from 2007 to 2011, and which record annual landform changes of the riverbed. In addition, images taken with an interval shooting camera were used to observe flood conditions of the riverbed from 3 July 2011 to 4 October 2011. Landform changes and/or sediment transport in the riverbed occurred almost every year from 2007 to 2011 during severe flood events. Channel migrations are generally caused not by lateral shifting with lateral erosion but by channels being buried with sediments and new channels being excavated. There are some stable spots at bars and/or islands in the active riverbed where only slight landform changes occur over a period of four or more years. At those locations, pioneer plants, especially Salix arbutifolia, germinate and grow to form young pioneer patches. When lateral erosion occurs, it causes the destruction and/or size reduction of patches. If a small seedling willow patch survives for several years, it becomes a grown patch and finally old isolated trees. Because each patch was established in a different year, patches in various age and size classes are found in the active riverbed. The fluvial geomorphic processes provide dynamic environmental diversity for pioneer species in the active riverbed and cause the destruction and re-establishment of vegetation. As a result, vegetation diversity is created in the active riverbed of the Kamikochi Valley.