People conceptualize various natural borders in very different ways. Their relationship to these borders can be determined through the use of cognitive maps. These are a kind of mental image of regions that individuals draw in their minds and then transfer to paper, thereby producing cognitive maps. This study investigated the symbolic power of rivers as borders using the delimitation of Slovenian regions. The results are based on an analysis of about six hundred cognitive maps produced by respondents from across Slovenia. The respondents were selected based on random sampling. The maps, which they drew manually, were scanned and digitized into vector format. This yielded four thousand polygons that were converted to raster format; various mathematical functions were then applied to define regional borders. These were compared with the former provincial boundaries from 1914. It was determined that rivers, despite not being significant obstacles in a region, are the clearest and most enduring delimiting feature in an area.