In Ukraine, the river's width ranges from 60 to 250 metres, with its average depth being 3 m (10 ft). The mean annual discharge at its mouth is 360 m³/s (12,713 ft³/s). The river freezes over from early December to early April, and is navigable from Novhorod-Siverskyi to its mouth, which totals about 535 km (332 mi).
The Desna River originates in the Smolensk Heights of Smolensk Oblast, Russia. The river's source lies eastsoutheast from the city of Smolensk and not far from Yelnya in a forest near the village of Naleti. The Desna then flows south through a low and swampy valley toward the city of Bryansk, where the river's right bank rises.
After its confluence with the Seim River near the Russian-Ukrainian border, the river then widens, splitting into numerous smaller branches. Its right bank declines again near the city of Chernihiv, and again near one of its tributaries, the Oster, where the Desna continues its course through a low, muddy plain until it finally reaches its mouth near Kiev at the Dnieper River.