Predicting how climate change will affect a species’ range, which is already shaped by environmental stress tolerance, is a central problem in eco-physiology, especially for invasive species. We address this problem using the globally invasive aquatic snail, Potamopyrgus antipodarum, as a model. Whether a site is likely for future invasion depends on P. antipodarum’s tolerance to the dissolved oxygen (DO) and temperature at that site. We previously investigated the effects of temperature and DO on respiration and locomotion to understand what conditions limit fitness related traits. We found that extreme temperatures limited both respiration and locomotion, but that hypoxia limited respiration to a greater extent than locomotion. Locomotion under warm, hypoxic conditions may be fueled anaerobically and thus, time limited. The current study draws on the physiological limits determined in the lab to see if limiting conditions already exist in the field. We predicted that high temperatures and hypoxia would co-occur with small populations. We monitored the temperature and DO conditions in three streams in different San Francisco Bay Area microclimates for one year. In each stream there were sites with dense and sparse snail populations. We found that sites with dense snail populations had the highest overall temperatures and were commonly hypoxic, with some exhibiting anoxia for several hours at a time. Sites with few snails exhibited mild conditions. Overall, the current environmental conditions in each region support snail populations, but future warming and hypoxia may constrain population growth at the warmest sites by reducing thermal safety margins.