First, three errors:
- The relative clause refers to people ('locals'), not things, so the relativizer should be who, not which
- They are multiple people, so the verb must be plural are, not singular is
The main clause is in the past tense (noticed), so the relative clause should probably be in the past tense, too—unless you mean explicitly that these locals are resting in the wintertime now, at some time after they noticed the ice.
... who were resting in the doldrums of off-season winter ...
Your rewrite of this participle clause (it's called that because the verb which heads it is a present participle) as a relative clause has similar meaning to the original, but not quite the same meaning.
The relative clause says the same thing about the locals—that they were resting during the winter when they noticed; but the resting is (as far as we can tell) incidental to the action of the main clause, which is their noticing the ice coating the walls of the gorge. The participle clause, however, suggests more strongly that it was in some sense because they were resting during the wintertime that they noticed the ice. Presumably, during the offseason doldrums they had a lot of leisure time on their hands when they could examine the landscape.