The rear floor is the low point in the interior. Leaks usually enter at the windshield or at the battery tray area and run to the back where they accumulate.
Remove your battery and tray and check under there. The impact may have opened a spot that was rotting. It could have also shifted the windshield causing an entry point. Use a garden hose to help you locate the source of the leak.
If it's the windshield, use Butyl Rubber Sealant between the car and the windshield gasket to seal it up. Put painter's tape on the car around the gasket to protect the paint and squeeze the sealant under the gasket just until it begins to come out. Then wipe up the excess with mineral spirits before removing the tape.
If it's the battery tray area, you may need to do a repair using POR-15 products to neutralize the rust, seal the area and repair any holes.
Good luck.
