Stream modification patterns in a river basin: Field survey and self-organizing map (SOM) application

In this study, stream modifications were surveyed in order to discover the relationships between geographical characteristics, human population distribution, and artificial stream alteration in the Nakdong River system, South Korea. Prior to this study, there was no comprehensive survey of stream modifications of the Nakdong River basin, even though the utilization of its water resources and ecosystem is recognized as an important issue. A total of 1655 stream sites were investigated by applying the Stream Modification Index (SMI), consisting of 12 parameters, comprising three characteristic factors of channels, land use, and levees (each characteristic factor contained four parameters). Those parameters were dichotomous (i.e. marked as 0 or 1), and a higher score of summing 12 parameters values (the SMI score) indicates a more modified state (maximum 12, minimum 0). This data was averaged in accordance with 265 unit catchments in the Nakdong River basin, and compared with population density, seven land coverage categories, elevation, and slope of each of unit catchments to discover general patterns of stream modification in the river basin by the application of a self-organizing map (SOM). A general tendency of increase in survey scores was observed in which unit catchments in urbanized area as well as high population density was found, and significant Spearman rank correlation coefficients were obtained for those relationship. However, though the statistical analysis exhibited significance, the relationship between survey results and socio-geographical information was unclear. SOM application clustered the 265 unit catchments into four groups on the map size of 9 × 6 plane (quantization error 0.329; topographic error, 0.000), such as catchments where streams were largely modified due to urbanization (cluster 4), relatively well preserved due to high elevation (cluster 2), moderately modified due to agricultural land coverage along with the main channel of the Nakdong River (cluster 1), and the remaining catchments with relatively moderately modified streams (cluster 3). The modification degree represented by the index scores was relatively high in which catchments in a highly urbanized area with large human population density exist, while scarce modification of stream occurred in relatively elevated and forested area. The results of this study suggest not only information and evidences of the general tendency of artificial stream utilization, but also the efficiency of SOM application to a basin-level characterization.