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

Erosion and deposition by flowing water generally follow simple rules relating 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 rules and the ensuing self-organization of the landscape. This paper discusses a cellular model of long-term evolution of a fluvial landscape. The model 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 moves from each cell to its lowest neighbor. Sediment is routed downslope according to a transport equation with the transport rate dependent on the elevation difference between two adjacent cells. The model allows both erosion 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 location and the whole process is repeated. Starting with a block-faulted landscape, over time a drainage network evolves. Sediment yield records of the drainage basins display a complex behavior, even though there are no external factors that would explain the variations in sediment yield. The complexity of sediment dynamics in the model arises from self-organization within the modeled system itself. This study is a first step towards separating the impact of this aspect of complexity on the sediment yield and depositional record from the impact of external factors associated with global change. (C) 2001 Elsevier Science Ltd. All rights reserved.