The Agency told the government that more needed to be done to prevent collapses such as that nearly suffered by Ulley Dam near Rotherham, north England, last summer following heavy rains.
The warning comes in a review from the Agency of last summer's heavy flooding. This will feed into the Pitt Review being carried out by the Cabinet Office into the floods.
The Agency claims in the report that under current regulations Ulley was found to pose little risk to life and property. This is because the risk assessments are based on the size of the reservoir rather than the danger they pose to the public.
"The events in June highlighted the very real risks to life and property should reservoirs fail," the report states. "The incidents at Ulley and elsewhere demonstrate that we need a better risk-based approach to reservoir safety. This means focusing on those reservoirs that pose the greatest risk to the public, even if they're not currently covered by the regulations."
Post-incident site inspections should become compulsory and be independently reviewed, the Agency added.
In its submission the Agency said it should be carrying out more dredging of rivers to alleviate flood risk. It said that it carries out £34M worth of maintenance, including dredging, along Britain's 12,800km network of main rivers.
"We will review the timing and frequency of certain river maintenance activities and explain clearly the reasons for our approaches to river maintenance to affected communities," the report says.
Ulley Dam investigation
Site investigations specified by Arup and carried out by Geotechnics started last month to check stability at Ulley Dam near Rotherham.
The investigation includes installation of vibrating wire piezometers in boreholes to monitor pore water pressure. Boreholes were drilled by a specialist slope-climbing rotary rig supplied by Geotechnical Engineering.
A combination of trenches and boreholes were used to examine the clay core and a later concrete extension to the core. The material in the shoulders of the dam was also examined.
"The site investigation is a health check on the embankment," said Arup associate Rachel King. "We were looking at stability, whether there were problems with seepage and gathering information for geotechnical analysis. The investigation works were successful."