The sandstone rocks on the banks of the Gauja and its adjoining rivers started forming 370 to 300 million years ago, during the Devonian period.

In some places, the bed is pebbly, forming boulder rapids: Kazu, Raiskuma, Rakšu, and Ķūķu.

The bottom of the river in Gauja National Park is 60 to 120 meters wide with a rapidly changing depth from 0.3 m to 7 m. The decline is 0.5 m/km.

Due to the fluctuations in water level, current speed, and special flow features, the Gauja River may be characterized as a rather non-homogeneous watercourse.

Usually the Gauja River freezes over in mid-December, and the ice starts moving in late March.