Spring floods are dated using ice scars on trees and shrubs. Marks on trees made by ice displaced by the wind indicate the flood levels. Once ice breakup is complete, waves erode forest edges. Tree tilting, dated by counting rings in compression wood, mark high lake levels at the beginning of summer. At submerged sites that are not subject to waves and ice, stress related to prolonged submersion results in narrow tree rings.
Trees located in the immersion zone show narrow rings compared to wider rings in unsubmerged trees.
Rings of submerged trees are denser than average. These high water level markers, as well as population recruitment showed by the age structure of shoreline trees and shrubs, which establish during intercalary years of low water levels, were used to date the extreme levels of two large lakes at the interface between the upper boreal and subarctic regions in northeastern Canada. During the past 200 years, the annual extreme levels have been increasing, becoming even more pronounced in the 1930s, and may be due to a century-long increase in snow precipitation. The summers seem to be drier. The 1930s, 1950s, and 1970s were marked by a high frequency of high water levels.