The influences of land-use changes on hydrology and riparian environment in a northern Japanese landscape

Temporal changes in a hydrological system and riparian ecosystem were examined with reference to land-use conversion in order to clarify the linkages between these two systems. First, the hydrological system of the Toikanbetsu River basin was divided into three components that measure water retention, inundation and conveyance. Variation in the hydrological system was expressed as a basis of delineating the three components and estimating their functions. The rainfall-runoff system was also examined using a model which can predict responses of surface-, subsurface- and base flows on rainfall intensity. Second, areas and fragmentation of the riparian forests, maximum stream temperature in summer and amount of coarse woody debris (CWD) were selected as parameters indicating the condition of the riparian ecosystem. Temporal changes in stream temperature and amount of CWD were estimated using multiple regression analysis and analysis of variance, respectively. The results indicated that the hydrological system has been altered since the 1970s, increasing flood peaks by 1.5-2.5 times and shortening peak appearance by 7 hours. Riparian forests have been disappearing since the 1960s due to extensive development of agricultural lands and river channelization. The summer maximum stream temperature increased from 22 degrees C in 1947 to 28 degrees C at present. The amount of CWD should substantially decrease with river channelization and associated forest cutting. Fish favoring cool water, such as masu salmon, could survive in 1947 although they are forced to migrate to cooler forested upstream tributaries now. The ecological systems were closely related to and distinctly altered by land-use. Finally, we propose a new perspective for understanding the two interrelated systems. Riparian ecosystems can be restored by restoring water retention and inundation functions, which also reduce the flood hazard generated by elevated flood peaks.