Moreton Bay's waterways will die within 20 years if more isn't done to protect the health of the bay, a leading Australian water scientist has warned.

Water sciences professor at Griffith University Jon Olley made the warning at the release of the 13th annual Healthy Waterways River Health findings on Wednesday.

Moreton Bay, seen from Moreton Island.

Despite improvements in some river catchments, the scorecard for Moreton Bay showed the environmental health of the bay had worsened over the past year.

Professor Olley, a member of the Australian Rivers Institute and the Healthy Waterways Scientific Panel, said the health of central bay declined from an A-minus to a C-plus over 12 months, while Bramble Bay, near Redcliffe, dropped from a D grade to a fail grade of F.