How far ahead can river levels be forecast?

Not as far as most people expect, and the honest answer is worth having before you go looking. Every forward number published for these rivers comes from a national or state authority, and the furthest any of them reaches is six weeks — as weekly means, not as a level for a named day.

These are the products the waterway and weather authorities publish. Waterlogged makes no forecast of its own and republishes none of theirs: every horizon below is a link to the body that issues it, and every number on the rest of this site is a measurement already taken.

Hours to two days

The state services run the short forecasts, gauge by gauge, and they are the only numbers anybody publishes for tomorrow morning.

Three to six days

Three national services reach the end of the week: Saxony on the Elbe, Hungary at Budapest, and the Bulgarian Danube authority on the border run.

Fourteen days

One product, one river. Germany's federal hydrology institute publishes a fourteen-day water level for the Rhine, one value a day, and for nothing else.

Six weeks, as weekly means

The longest published horizon on any of these rivers, and the one this site reads every morning. It is a weekly mean rather than a level for a named day, which is a different thing from a forecast and is worth understanding before you read one: it says roughly where the week sits, not what Tuesday will be.

Past six weeks there is nothing, and that is not a gap in this site

Past about six weeks nobody publishes a water level for a named day, and a site that gave you one would be inventing it. What exists at that distance is the record — what the water has done in that month across the years on file — and what your cruise line has committed to in writing if the river does not cooperate.

Two things worth knowing about the numbers themselves

A weekly mean is not a level for a day

The six-week outlook is the longest thing published on these rivers and it is the one most easily misread. It gives a figure for a week, not for a Tuesday: a week whose mean sits comfortably above a low-water reference can still contain a day that does not. Read it as the shape of the month rather than as the answer for your sailing date.

A forecast for a lock-held river is a different question

On the Main, the Moselle, the Main–Danube Canal and the Austrian Danube the depth is held by barrages, so a low reading closes nothing and the state services only forecast those gauges once the level is rising. What binds a cruise on that water is high water and lock operations, and the forward numbers published for it are about exactly that.

Linking is not endorsement. The publisher controls its own page, its own values and its own terms.