Why do rivers on the East European Plain tend to be long and slow-moving?

Rivers on the East European Plain are long and flow sluggishly primarily due to the region's topography. The area is characterized by an uninterrupted, low-lying landscape that remains almost entirely below 1,500 feet (450 meters) in elevation, extending from the Black Sea to the Arctic.21

Because the region is underlain by a rigid platform of ancient rocks that has not been sharply deformed, the terrain lacks the steep gradients necessary to generate high-velocity water flow. Consequently, the rivers are considered "mature," meaning their valleys are well-graded, allowing them to traverse vast distances across the flat plain at a slow, steady pace as they drain toward five different seas.21