Sweden's national weather agency SMHI issued a class-three warning (the most serious kind on a scale from one to three) for the Gävleborg region, in particular the area around the Norrala river.
It said river levels were "extremely" high after the sudden arrival of spring and melting snow. Rivers only rise to this level on average every 50 years, it said.
Fire and rescue services stepped up their preparedness and Sweden's civil contingencies agency MSB was called in to assist in the region over the weekend to help build temporary flood defences.
"In Söderhamn, several measures have had to be taken, such as helping people to pump their flooded basements," Gävleborg county administration's crisis manager Per-Åke Svensson said.
On Tuesday morning SMHI also had a similar, but less serious, class-two warning for flooding in place for Dalarna county west of Gävleborg and a class-one warning for most of central Sweden.
The flood risk is expected to peak mid-week and then subside as the melting snow settles.