 By the mid-19th century, there were three main conventions, one following the Don, the Volga–Don Canal and the Volga, the other following the Kuma–Manych Depression to the Caspian and then the Ural River, and the third abandoning the Don altogether, following the Greater Caucasus watershed to the Caspian.The question was still treated as a controversy in geographical literature of the 1860s, with Douglas Freshfield advocating the Caucasus crest boundary as the best possible, citing support from various modern geographers.In Russia and the Soviet Union, the boundary along the Kuma–Manych Depression was the most commonly used as early as 1906.