Food habit studies in the Amazon emphasize the consumption of fish and manioc (Manihot esculenta) flour as basic diet in different ethnic groups, while little is known about human plant-related diet dynamics during the hydrological regime. In a scenario of food transition in rural Amazonia, with insertion of industrialized items in the diet, traditional food and consumption of regional products have undergone transformations, affecting the autonomy and lifestyle of traditional populations. Considering this, the objectives of the current case study were: (1) to contextualize plant-related eating habits in five Riverine communities on the Lower Purus River and (2) to characterize the dietary dynamics of plant eating according to the hydrological regime, considering the provenance of food items. We conducted interviews and sample collection in both low- and high-water seasons. We calculated species richness and diversity per meal and season and used multivariate analysis to access differences in plant consumption between seasons. The greatest richness and diversity of plants is consumed in the flooding season and mainly as snacks, the meal most susceptible to being replaced by industrialized foods. Despite a significant difference in diet between seasons, the basic array of plants consumed is similar in both seasons, with availability topped up by external purchases. This tendency deserves attention and action from public policies aimed at providing food security in the region.