The most spectacular part of Tulcea County is the Danube Delta, which occupies about 1/3 of the entire surface and is located in the North-East side of the county. The Delta has three main branches: the Sulina branch (the only one navigable by large ships) in the middle, the Chilia branch in the North (the border with Ukraine) and the Sfântu Gheorghe branch (Saint George) in the South.
The Danube River flows around the county in the West and North side. In the West there is the Măcin Branch which flows on the East side of the Great Brăila Island. From Smârdan (on the opposite side from Brăila) to Pătlăgeanca (close to Tulcea), the Danube has only one big flow of water, around which there are an immense number of lakes and small channels.
In the center of the county there is the Casincea Plateau and the Măcin Mountains—the remains of a prehistorical mountains range (more than 400 million years old), with the greatest height at about 400 m.