1 National Key Laboratory of Petroleum Resources and Engineering,China University of Petroleum(Beijing),Beijing 102249,China; 2 College of Geosciences,China University of Petroleum(Beijing),Beijing 102249,China
Abstract：Grade refers to the state of a river where sediment is conveyed without net deposition or net erosion but completely bypassed through the river. The concept of grade is fundamental to geomorphology,geology and stratigraphy,because it represents the critical condition between river aggradation and degradation. According to conventional views,rivers will spontaneously approach the theoretical graded profile and finally reach it,given constant external forcings such as stable base level and steady sediment supply. The conventional notion of grade is valid for local scale of the alluvial system,but not for system scale of the whole alluvial system from a source-to-sink perspective. This is because the conventional notion seldom considers the free boundary condition of the alluvial system(i.e.,the upper and lower boundary of the alluvial system can move freely). For example,extension of the alluvial system by delta progradation at the downstream end of the river is inevitably accompanied by alluvial aggradation. This indicates that in moving boundary conditions,the necessary condition for the alluvial river to attain the state of grade at system scale is base level fall rather than base level standstill. Physical and numerical experiments further verified that the alluvial river can reach system-scale grade by three ways. (1)Forced grade,which can be realized in fixed downstream boundary conditions,for example,the downstream end of the alluvial river cannot extend freely by extreme deep basin water depth. In this case,the alluvial river grades to stationary base level. (2)Autogenic grade,which is realized in full moving boundary conditions and the basin floor slope equals that of the alluvial system. In this case,constant rate of base level fall is necessary. (3)Allogenic grade,which is realized in full moving boundary conditions and the basin floor slope is steeper than that of the alluvial system. In this case,slowing down of the base level fall in a particular pattern is necessary. Genetic stratigraphy which highlights the stacking process of strata should fully take the three ways of grade into account,by which the interpretation of stacking of strata will be clearer.
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