Barbarian Invasion Era
The history of Latvia between 400 AD and 799 AD
During the so-called Era of the Barbarian Invasion (400—800 AD) did the Slavs begin to move northwards from the steppes of Southern Russia. This migration began under the pressure of the sub-Black Sea Goths and several Tartar-Turk tribes. The Slavs moved into the woodlands inhabited by some of the extreme Eastern Balt tribes. The total area inhabited by Balts at that time was very wide and covered White Ruthenia, extending deep into Central Russia as far as Tula and Chernigov, to the regions where the rivers Dnieper, Oka, Volga and Daugava have their sources.
Under the pressure of Eastern Slays (the Russians), one Balt tribe, the Latgali, moved down the River Daugava (Western Duna) and joined their kinsfolk in Latvia, gradually pressing the Estonians further north.
Reference: Latvians.com