We analyzed population trend data during 1991-2004 for the Bruneau hot springsnail (Pyrgulopsis bruneauensis) from rangewide hot spring surveys completed along 4 km of Bruneau River. The objectives of this study were to analyze population trends and assess other measured parameters, such as water temperature, specific conductance, and groundwater decline, for changes that may influence population trends. We used simple linear regression to determine significant relationships between measured parameters. Total numbers of hot springs surveyed in the 4-km reach are declining at a rate of ~5 hot springs per year, and numbers of hot springs containing P. bruneauensis in the same reach are declining at a rate of 3.75 per year. We found that the decline of hot springs with and without P. bruneauensis was significantly related to the decline of the thermal aquifer which underlies this area.