It is well known that the population structure of plants in permanent plots change from year to year. Species abundances probably shift with time due to both species biology and variation in environmental factors. The main purpose of the study has been to describe and analyse long-term variation in the abundance (coverage) of blister sedge Carex vesicaria and reed canary grass Phalaris arundinacea in relation to time and space after a permanent lowering of a lake water level and exposition of new areas for species colonization. The study area lies in outer edge of a freshwater fluvial delta in the northern part of the Myrkdalen lake (229 m a.s.l.) West Norway, which was exposed after a permanent 1.4 m lowering of the summer water level in 1987. A 40 m long transect with ten 0.5 m × 4 m plot was established on the lower parts of the exposed delta in 1987. Study plots were situated at mean heights between 13 and 92 cm above the new summer water level. Floristic composition of the plots was monitored and analysed from 1988 to 2020, a total of 19 times. The plants show major differences in establishment, growth, and long-time dynamics in the permanent plots. The coverage of C. vesicaria and P. arundinacea was highly variable over the study period. Variations in coverage were related to time using regression analyses. The trends were mostly best described by cubic functions. Both species showed relatively low coverages between 18 and 26 years after the lake water drawdown, but increased again over the last study periods. Maximum coverage of the species was in time gradually separated by elevation. C. vesicaria became dominant in plots less than ca. 50 cm above the mean summer water, and P. arundinacea reached higher coverage above that elevation. The study indicates that elevation, as a proxy for flooding duration, and not aboveground species competition was the main factor for this difference in distribution.