Saxicolous lichen trimlines are relatively level and distinct transition zones found on bedrock-lined shores. They occur as a result of disturbance to the rock lichen community, typically due to high water events. Trimlines can inform about previous high water conditions, and in conjunction with lichenometry, can be dated. In wetlands which fluctuate in water level, dated trimlines may be useful in reconstruction of the timing and magnitude of former high water. We assessed seven common northern saxicolous lichen species over a period of 270 days to determine the period of immersion required for mortality. Overall mortality rates increased after about 30 days of immersion. At 90 days, mean mortality was about 68%; at 180 days, mean mortality was about 91%; total mortality of the immersed lichens was observed at day 270. There were large interspecific differences in susceptibility to immersion. Phaeophyscia sciastra was the least susceptible to immersion and Xanthoparmelia somloënsis was the most susceptible to immersion. A prolonged period of high water (months, rather than days) may be required to form a trimline in a northern delta. A northern saxicolous trimline might remain visually distinct for several decades in the absence of disturbance.