Self-organization in fluvial landscapes: sediment dynamics as an emergent property

Erosion and deposition by flowing water generally follow simple rules relat ing the rates of erosion and deposition to slope angle and other variables. When these rules are applied at small scales, the resulting landscape has large-scale properties which are apparent in its morphological attributes, e.g., drainage network configuration, and in its functional attributes, e.g ., sediment dynamics. These emergent properties are not part of the basic, small-scale rules but, instead, result from repeated application of these r ules and the ensuing self-organization of the landscape. This paper discuss es a cellular model of long-term evolution of a fluvial landscape. The mode l is started by applying rainfall to a square group of cells of random size and at a random location within a grid. Erosion takes place as the water m oves from each cell to its lowest neighbor. Sediment is routed downslope ac cording to a transport equation with the transport rate dependent on the el evation difference between two adjacent cells. The model allows both erosio n and deposition of sediment, depending on the difference between sediment input and output of a cell. When all runoff has been routed across the edge of the grid, a new rainstorm with a random area is applied at a random loc ation and the whole process is repeated. Starting with a block-faulted land scape, over time a drainage network evolves. Sediment yield records of the drainage basins display a complex behavior, even though there are no extern al factors that would explain the variations in sediment yield. The complex ity of sediment dynamics in the model arises from self-organization within the modeled system itself. This study is a first step towards separating th e impact of this aspect of complexity on the sediment yield and depositiona l record from the impact of external factors associated with global change. (C) 2001 Elsevier Science Ltd. All rights reserved.