Landscape Ecology 14: 543–556, 1999. © 1999 Kluwer Academic Publishers. Printed in the Netherlands. 543 The influences of land-use changes on hydrology and riparian environment in a northern Japanese landscape Akiko Nagasaka 1∗ and Futoshi Nakamura 2 1 Hokkaido Forestry Research Institute, Koshunai, Bibai, 079-0198, Japan 2 Department of Forest Science, Faculty of Agriculture, Hokkaido Univ., Sapporo 060, Japan ( ∗ author for correspondence, e-mail: pako@hfri.bibai.hokkaido.jp) (Received 20 June 1997; Revised 20 October 1998; Accepted 27 February 1999) Key words: channelization, Coarse woody debris, hydrology, land-use, riparian ecosystem, stream temperature Abstract 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 con- veyance. 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 ◦ C in 1947 to 28 ◦ 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. Introduction In Japan, there has been intense concern about the degradation of riparian ecosystems as flood control works have expanded in the past ten years. New management perspectives which involve flood control and conservation of riparian ecosystems are urgently needed. This problem cannot be solved simply by con- servation at the riparian fringe. Rather, the restoration of the hydrological and ecological watershed is also required. In general, the possibility of flooding in- creases as the result of human land use conversion, because the increase of surface-water discharge causes a shortening of the arrival time of flood peaks and an increase in flood peak-flow (Takahashi 1971). With more intensive land-use, the need for flood preven- tion increases resulting in a deterioration of riparian ecosystem. The rainfall-runoff process is an integrated hy- drological system within a landscape, and land-use development substantially alters the spatial hetero- geneity of landscape elements, which in turn changes the rainfall-runoff system. For instance, shortening of the river channel, construction of dikes, implementa- tion of open and underground drainage ditch systems are all common engineering works used in agricul-