Whats on the river, these objects Birna?
Structures used for controlling the log driving. Called variously log piers, boom piers, stone cribs, and probably more designations. Norwegian name is 'lensekar'. They measure around 15 by 15m and enclose many tons of rocks.
This river, the largest in the country, was extensively used for timber rafting and log driving until the late '70s. At the peak periods one could literally walk across the river. However the danger of being crushed by logs was significant and the drivers ('river pigs') had an elevated mortality rate.
Here is a closer look at these structures.