In the past 20 yr, populations of Cicindela hirticollis Say (Coleoptera: Cicindelidae) that inhabit river shorelines have declined dramatically. These habitats have routinely flooded in winter and spring historically, but they have been altered by damming and controlled water releases for irrigation and power generation. We tested the ability of C. hirticollis larvae from two river and one bayshore population to survive immersion in severely hypoxic water. This is the first report of population-level differences among insects in immersion survival and likely relates to exposure to different flooding regimes. The larvae from the Chesapeake Bay population survived ≈3 d of immersion, and those from river populations survived about a day longer. Despite survival differences between riverine and seashore populations, recovery times after exposure to severe hypoxia were comparable. Second and third instars from the seashore population had similar survival at 9.0 and 16.5°C. Survival times of larvae more than doubled under aerated conditions. Adults survived >30 h of immersion in severely hypoxic water, substantially longer than reported for other tested tiger beetle species. Although riverine populations survive longer periods of immersion, dams cause habitats used by riverine populations of this species to be inundated for weeks at a time, far longer than larvae were able to survive under hypoxic or aerated conditions in the laboratory. Thus, alteration of flooding regimes and subsequent larval habitat immersion is probably a major cause of the observed decline of riverine populations of C. hirticollis. Moreover, these data represent the first report of significant physiological differences among populations of an insect species exposed to different frequencies of immersion and thus have both important experimental and evolutionary implications.