Why does the Ob River have a gentle gradient across western Siberia?

The Ob River exhibits a gentle gradient across western Siberia primarily because it flows through the West Siberian Plain, which is one of the world's largest regions of continuous, exceedingly flat land.1

This vast plain is characterized by prolonged geological subsidence and is composed of horizontal deposits that have accumulated over millions of years. Because the region is exceptionally level and featureless, the river lacks significant elevation changes, resulting in a minimal gradient, particularly as it traverses the taiga in its middle course. The landscape's flatness, combined with the presence of extensive marshlands and poor drainage, contributes to the river's slow, meandering flow through a complex network of channels.12