Because the importance of the process of capture, or piracy, in the formation of stream networks is difficult to evaluate by field or map studies, an indirect approach is used in this paper to investigate capture, through the use of a simulation model involving capture within rectangular stream networks on a square matrix. The simulation rules make the probability of capture of a stream by a lower adjacent stream proportional to the advantage in gradient of the potential path of capture between the streams compared to the present gradient of the higher stream. Stream elevations are assumed to be defined by the same type of pattern observed in natural stream networks, that is, a linear relationship between the logarithms of gradient and drainage area. The slope of this relationship, Z, is variable in nature and is the main adjustable parameter in the simulation model. Simulation of capture must start from assigned initial network patterns; random walk networks and parallel drainage are among those used for initial networks.
For a given value of Z, the statistical properties of networks (for example, stream numbers, length and area ratios, and shape factors) formed after repeated captures are nearly the same for a wide range of assigned initial networks. However, when the value of Z changes during capture, the statistical properties of the resultant networks may depend upon the type of change, so that properties may be partially inherited from earlier stages of basin evolution.
Both the networks simulated by capture and natural networks have similar slight deviations from topological randomness. The capture simulations more closely predict many properties of natural networks than do completely random methods of simulation, such as the random walk. In addition, several parameters in the capture-simulated networks exhibit a consistent trend with respect to the parameter Z that appears to occur also in natural networks. These correspondences between the capture model and natural networks suggest that capture may be an important natural process. However, capture should have its greatest relative importance in early stages of drainage basin evolution.