A study was conducted to assess the vertical accretion of sediment in the Mud Lake impoundment of Sand Lake National Wildlife Refuge, located in Brown County, South Dakota. Sediment cores were collected from the Mud Lake impoundment during 2000 for determination of vertical accretion rates, which were estimated using cesium-137 and lead-210 isotopic dating techniques. These data directly support the associated publication “Sedimentation Rates in the Marshes of Sand Lake National Wildlife Refuge, South Dakota” which is referenced within the Metadata.