It was unclear if the man managed to get out of the water at a different point, and the force urged him to contact police if that was the case.
Anyone else with information or who saw a man in the water has been urged to also speak to the force.
A spokeswoman for Northamptonshire Police said: “Searches will resume this morning with a specialist team.
“The person who reported the incident was still at the scene when we arrived and has been spoken to.
“This is being treated as a high-risk missing person investigation.”
