Nearly 200 people lost their lives in Germany and Belgium.
Europe is on Tuesday to pay tribute to the 196 lives lost in devastating floods.
Belgium honoured its 31 dead with a minute of silence at midday as part of its day of "national mourning". All flags are flown at half-mast for the day.
King Philippe and Prime Minister Alexander de Croo observed the minute of silence in Verviers, near Liege, where water from the torrential rain unleashed considerable damage after meeting with rescue workers and victims.
European Council President Charles Michel, a former Belgian Prime Minister, joined in silence with other EU officials at the bloc's headquarters in Brussels.
"Our thoughts are with their (the victims') families and friends," he wrote on Twitter. "The EU stands by you at this difficult time."
About 70 people are still unaccounted for across Belgium following the floods of July 14 and 15, according to the crisis centre. The figure has gone down significantly over the past 48 hours as telephone lines have been restored.
The Vesdre valley, a tributary of the Meuse that burst its banks under the effect of the torrential rains and the saturation of a dam, is home to at least half of the victims, according to local politicians.
European Union environment ministers, who are meeting in Slovenia on Tuesday, will also observe the minute of silence.
German Chancellor Angela Merkel will meanwhile visit North Rhine-Westphalia on Tuesday to talks to rescuers and meet with the region's Minister-President Armin Laschet.
Germany has borne the brunt of the floodings with 165 fatalities recorded so far — 117 in Rhineland-Palatinate, 47 in North Rhine-Westphalia and one in Bavaria.
It comes a day after she promised to improve the country's disaster warning system, severely criticised since the floods.
A German government spokeswoman conceded that "the experiences we have had with this disaster show that we have to do more and better."
Civil protection president Armin Schuster called on public radio for "the return of the good old sirens", so as not to rely solely on digital tools but the siren system — a legacy of the Cold War to warn in case of a nuclear attack in particular — proved ineffective in September 2020, when a major test at national level turned into a fiasco.
Some of the sirens did not go off due to technical problems, and some municipalities removed them altogether, deeming them useless.
Nearly 750 people were also injured in Germany because of the floods. Nearly 200 are still missing.