Seasonal? Norway must have some huge reservoirs that don't freeze.
The ice is only a foot or so thick. What Norway must have are reservoirs where surplus power is used to pump water uphill into a storage facility where it later can be released for power generation.
It adds to the problems/costs, but doesn’t make it impossible.
In Norway, hydropower accounts for nearly 99% of the total electricity production. To meet winter demand, storage schemes are implemented in tandem with run-of-river schemes to a large extent in cold region hydropower systems. In these systems, ice creates operational constraints during winter that can lead to reductions in power production. The problems occur in the various phases of the ice regime, mainly due to frazil ice, ice runs, and ice jams. Counteracting these ice problems is usually a difficult task that involves expensive measures and possibly lost production.
Read More:
http://ascelibrary.org/doi/abs/10.1061/(ASCE)CR.1943-5495.0000059
With all that cheap energy they could just heat them...:-) Would have further benefits as well. "Hey Olaf, it's finally winter - let's go for a swim in the reservoir."