Thirty-four people died as a result of floods in the Kastamonu district which includes Bozkurt, and another six people died in Sinop, the disaster and emergency management directorate (AFAD) said.
In one collapsed building along the banks of the swollen river, 10 people were still believed buried. The rapid floodwaters appeared to have swept away the foundations of several other apartment blocks.
Relatives of the missing, desperate for news, were nearby.
"This is unprecedented. There is no power. The mobile phones were dead. There was no reception. You couldn't receive news from anyone," said Ilyas Kalabalik, a 42-year-old resident.
Kalabalik was surrounded by residents who were asking each other whether anyone had any news about missing people.
"My aunt's children are there. My aunt is missing. Her husband is missing. Her twin grandchildren are missing. The wife of our building manager is missing along with their two children," Kalabalik told Reuters.
The floods brought chaos to northern provinces just as authorities were declaring wildfires that raged through southern coastal regions for two weeks had been brought under control.
About 45 cm of rain fell in less than three days in one village near Bozkurt.
Torrents of water tossed dozens of cars and heaps of debris along streets, destroyed bridges, closed roads and cut off electricity to hundreds of villages.
The small town of Bozkurt lies in a valley along the banks of the Ezine river in Kastamonu province, 2.5 km from the Black Sea.