The $650 million Kingsford Smith Drive project will be delayed by up to one year after what was described as a "retaining wall" was found in the Brisbane River.

The road project was originally expected to be completed by the end of 2019, but on Thursday while lord mayor Graham Quirk was announcing the nearing completion of the 1.2 kilometre riverwalk, it was revealed the road would be delayed by between six months to a year.

The joint Brisbane City Council and federal government project aims to increase the capacity of Kingsford Smith Drive, reduce traffic congestion and improve the safety of pedestrians and cyclists.

Lord mayor Graham Quirk and councillor Amanda Cooper at Kingsford Smith Drive. Credit:Jocelyn Garcia, Fairfax Media

Cr Quirk said geotechnical work by a contractor had revealed a section of the project was causing a problem for council.