A New View of the Graded River: Theory and Flume Experiments

Tetsuji Muto¹ and John Swenson^2
1 Nagasaki University, Nagasaki, Japan
² University of Minnesota Duluth, Duluth, MN

A river which conveys sediment downstream without net deposition or erosion through a series of reaches is referred to as being graded or in equilibrium. The fluvial grade concept has long been a subject of debate and survived centuries of discussion. It is still uncertain if such a graded river can exist in nature and how it could be recognized presently or in ancient strata. We here challenge the classical view, by means of theoretical modeling and flume experiments, and suggest a new view of the nature of graded streams. Two settings of basement slope are assumed here. One is a uniform slope of shelf; the other is of a submerged alluvial slope onto which a river delta downlaps. Theoretical examinations suggest that (1) the river attains and sustains a graded condition during sea-level fall in any of the slope settings, and (2) the patterns of sea-level fall to sustain the grade are different by the slope settings. In the shelf slope setting the rate of fall has to reach a particular pattern of deceleration, while in the submerged alluvial slope setting the sea-level fall is required to remain constant. With respect to the latter setting, multi-storied profiles of the graded river can be constructed with asymmetric cycles of sea-level fluctuation that is composed of very rapid rise and subsequent constant slow fall. These predictions were proved with flume experiments in which two-dimensional river deltas were constructed.

AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005