This is the largest beaver dam I have seen. According to the only authoritative source I have on beavers, E. R. Warren's 1927 book The Beaver, beaver dams could be from a few feet to thousands of feet long. He mentions one dam that was 2140 feet (about 700 m) long.
Warren explains that in the beginning of a dam, beavers lay the sticks and brushes longitudinally. Only after a dam is completed may they put sticks on it transversely. In this dam most sticks appear to be positioned longitudinally.
This dam is in Black Hill Regional Park in Germantown, Maryland, in an area where beavers have been active for several years. I will be posting more beaver-related pictures in the future.