Geomorphic evolution of the Yellow River Delta: Quantification of basin scale natural and anthropogenic impacts

The intensified impacts of both natural and anthropogenic processes in river basins on the sustainabilities of river deltas worldwide have necessitated serious international socioeconomic and environmental concerns. The Yellow River Delta (YRD), which formed within a relatively weak coastal dynamic environment, provides an excellent opportunity for a quantitative study of basin-scale natural and human influences on deltaic transformation. An examination of long-term bathymetric data demonstrates that the annual volumetric change of the YRD has experienced a statistically distinct declining trend during 1977-2005 with substantial inter-annual variations. Consequently, the decadal geomorphic evolution of the YRD has successively undergone three stages, namely, quick, stable and slow accumulation stages. Taking the fluvial supply as a link in combination with long-term hydro-meteorological data, the geomorphologic processes of the YRD are closely associated with the rainfall, air temperature and water diversion within the Yellow River catchment. A significant quantitative relationship exists between the deltaic land accretion and basin controls, indicating that annual morphological change will decrease by 4 x 10(8) m(3)/yr with every decrease of 100 mm/yr in annual precipitation, decline by 2.49 x 10(8) m(3)/yr with every increase of 1 degrees C/yr in annual air temperature, and diminish by 1 x 10(8) m(3)/yr with every increase of 100 x 10(8) m(3)/yr in annual water abstraction. Further non-dimensional analysis reveals that 50.55%, 36.26% and 13.19% of the inter-annual variation of the morphological change can be attributed to inter-annual variations of the precipitation, air temperature and water diversion, respectively. Natural environmental changes can account for 86.81% of the overall variations, while human-induced changes can explain the rest. Moreover, the contributions from rainfall, air temperature and water diversion to the decadal landform evolution transition from quick accumulation to stable accumulation were estimated as 46.59%, 35.23% and 18.18%, respectively, and their respective contributions to the transition to the subsequent slow accumulation stage were 2.09%, 92.67% and 5.24%. The natural contributions to the inter-decadal shifts were calculated as 81.82% and 94.76%, which are much greater than the respective human-related contributions of 18.18% and 5.24%. Our quantification results highlight that since the late 1970s, the changes of natural environment throughout the watershed have played a strikingly important role in both the inter-annual and inter-decadal changes of the sedimentary processes of the YRD. This study provides valuable quantitative references for the validation of basin-delta process-based research and for the sustainable development of the YRD. Furthermore, the YRD can be regarded as a typical case for deltaic systems that are currently being subjected to catchment-scale natural and anthropogenic influences, thereby suggesting the direction of future research.