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We examined soil properties and accumulation rates over the last ca. 100 years in four swamps and four marshes in southern Canada where these wetlands are common but under increasing anthropogenic pressure. One 50‐cm long core was collected from each wetland and analyzed to determine bulk density (BD) and organic matter (OM). Lead‐210 and cesium‐137 dating were used to estimate sediment accumulation rates. In the datable portion of each core, we determined the organic carbon (Corg), nitrogen (N), Corg/OM, Corg/N, Corgdensity, Corgstocks, and Corgaccumulation rates. All parameters but one—Corgaccumulation rates—were significantly different in swamps compared to marshes and between seasonally wet soils and those regularly flooded. Since 1950, Corgstocks varied from 6 to 13.6 kg m−2with ≈23% more Corgin swamps than in marshes. When hydrology and deeper soils where considered, Corgstocks in regularly flooded wetlands were 60% higher than those of seasonally wet systems, emphasizing the role of hydrology in building up C stocks long‐term. Our measurements are within the wide range reported for similar systems in North America. The average rate of Corgaccumulation in the studied wetlands (112 ± 87 g m−2yr−1) is ≈50% higher than that of oligotrophic/ombrotrophic peatlands, but ≈47% lower than in tidal marshes from eastern Canada. Our study lends support to the case for wetland restoration through re‐wetting as an important nature‐based climate solution for mitigation of GHG emissions in areas where they were drained for agriculture and other purposes. Wetlands are important ecosystems because they have an enhanced capacity to extract carbon dioxide—a greenhouse gas—from the atmosphere and store it in the ground as partly decomposed plant carbon. This ground storage is vital because it keeps carbon dioxide from increasing in the atmosphere where it causes global warming. Among the Earth's wetlands, freshwater marshes and swamps from temperate climates are among the least understood. To address this limitation, we conducted a study in southern Canada, where these wetlands are common but under increasing pressure from farming and urban development. Our results indicate that while marshes (dominated by grasses and sedges) form mostly mineral soils with less carbon stocks than swamps (vegetated with shrubs and trees), which form organic soils, the rate at which they accumulate carbon in their soil is similar. We also show that the wetlands we studied are 50% more effective in storing carbon than the cold climate bogs (vegetated by mosses), but they are less effective than salt marshes which form along the shores of temperate seas. We suggest that if freshwater marshes and swamps are protected, they can be important natural solutions in the mitigation of carbon dioxide emitted by human activity. Carbon stocks in eight nutrient‐rich freshwater wetlands ranged 18–40 kg m−2, with organic soils averaging ≈60% more than mineral soilsMean carbon accumulation rate over the last 100 years was 112 g m−2yr1, with no significant differences between swamps and marshesCarbon accumulation rates were higher than in oligotrophic/ombrotrophic wetlands (bogs) but lower than in tidal salt marshes Carbon stocks in eight nutrient‐rich freshwater wetlands ranged 18–40 kg m−2, with organic soils averaging ≈60% more than mineral soils Mean carbon accumulation rate over the last 100 years was 112 g m−2yr1, with no significant differences between swamps and marshes Carbon accumulation rates were higher than in oligotrophic/ombrotrophic wetlands (bogs) but lower than in tidal salt marshes