This study aimed to elucidate the environmental factors that determine spatiotemporal fluctuations of populations of the freshwater snail Kawanina (Semisulcospira libertina) in two streams in Higashi-Hiroshima City based on a field survey using a trap designed for sampling regardless of the individual snail size or sediment. The changes from May to November in the semi-natural stream Kadowakigawa were investigated, and the number of individual snails captured by traps varied greatly depending on the site throughout the period. Differences in the numbers of captured snails could not be explained by differences in water temperature, water flow, BOD, pH, or riverbed materials between sites. However, the number of snails tended to be stable, and small individuals (< 3 mm) were frequently captured in the site with a stony riverbed. In the Hannogawa, a concrete-lined stream, only a very small number of Kawanina were captured by the trap compared to the Kadowakigawa, and only one young snail was captured by the trap during the investigation period, so the snail reproduction in this stream was considered to be unsuccessful. Quantitative studies considering the size structure of the population are important in assessing the suitability of a stream for the habitat of the snail. The “Kawanina trap” can be a useful tool for such research.