In the context of global climate change driven by growing greenhouse gas concentrations in the atmosphere, the study of carbon deposition processes in natural environments is particularly relevant. The bottom sediments of water bodies, which act as long-term reservoirs of organic matter, play a significant role in the global carbon cycle. This article presents the results of a study on sedimentation and carbon accumulation processes in the sediments of 12 lakes. These lakes were formed by the separation of bays from the Kuibyshev Reservoir, located within the Saralinsky section of the Volga-Kama State Nature Biosphere Reserve and its buffer zone (Middle Volga, Republic of Tatarstan). The study identified the spatial distribution patterns of the sediments that have accumulated in these lakes since impoundment, as well as their thickness, which ranges from 5 to 48 cm. The sedimentation rate in the studied reservoirs was found to depend on their bed bathymetry, increasing from 0.8–3.0 mm/yr in areas shallower than 1 m to 3.9–7.2 mm/yr at depths exceeding 2 m. This corresponds to an annual mass accumulation rate of 680–3186 g/m2. The organic matter content in the sediments, which are predominantly composed of sandy silt and silty sand, is relatively low, ranging from 0.2 to 12.1 %. A clear relationship was observed between water depth and the organic carbon accumulation rate, which averaged 29, 53, and 66 g C m-2 yr-1 for depths of <1 m, 1–2 m, and >2 m, respectively. The highest carbon accumulation rates are typical for the deep water zones of most lakes, as well as for some shallow detached lakes that have intensive macrophyte growth, reaching up to 89 g C m-2 yr-1. Carbon stocks in the sediments per unit area of the lake bed also increase with depth: from 22 t C ha-1 in the shallow zone and 28 t C ha-1 at depths of 1 to 2 meters to 44 t C ha-1 in the deep-water zone. This corresponds to 79–161 t ha-1 in CO2 equivalent. Most of the terrigenous and authigenic carbon deposited in the sediments during the annual cycle returns to the biogeochemical cycle via ascending flows of gases (carbon dioxide and methane) from the lake bottom.