<p>Northern lakes provide habitat for wildlife, regulate biogeochemical cycles, and supply subsistence resources for local communities. Monitoring long-term changes in these lakes is crucial to understanding how human activity and climate change affect these ecosystems. However, multidecadal trends in northern lake area are highly uncertain, with different studies reporting directionally opposite trends over the same region. Here, we examine the sources of differences between lake area estimates and short-term lake area trends derived from one Sentinel-2-based and two Landsat-based surface water products across five northern study regions. We show that differences in the magnitude and direction of regional lake area trends are related to systematic between-product differences in surface water detection in dry vs. wet years, with larger discrepancies in dry years. In some regions, these between-product differences were substantial enough to result in directionally opposite short-term trends (2016–2021), providing an explanation for disagreements in long-term (decadal-scale) studies. These between-product differences in wet vs. dry years reflect how the products classify mixed shoreline and other ambiguous pixels, which are more prevalent in regions with small, shallow lakes and aquatic vegetation. Resolving discrepancies in long-term trends will require new technologies and methods designed to differentiate between water, land, and inundated vegetation.</p>