These plots monitor Rhine water level as it passes through Bonn and put it in historical context.

Few explanations:

1. Daily levels (black line) come from the German Federal Institute of Hydrology (BfG). The record starts in 1960, but it is only updated every few weeks.
2. Live levels (green line) come from the Bonn gauge itself (PEGELONLINE, WSV): one reading every 15 minutes for the last month. This fills the gap at the end of the BfG record.
3. Both sources measure from the same zero point, and on the days when both have data they agree to about 0.1 cm, so neither is adjusted.
4. The red lines show what is normal for the time of year, worked out from the years 1990 to 2021.
5. For each day of the year I did not use that one date alone. I used every value within 5 days either side, i.e., an 11-day window. That gives about 350 values per day instead of about 32. This matters for the outer percentiles: with only 32 values, "P01" and "P99" are just the single lowest and highest readings, which jump around from one day to the next.
6. Old data are corrected. On 1 November 1979 the gauge zero was moved down by exactly 1 metre. This means readings before that date are 1 metre too low. I added 100 cm to them, so the whole record can be compared directly. Without this correction the 1960s and 1970s look far drier than they really were.
7. When you select more than 3 years, the red lines and the black anomaly line are hidden, because otherwise the plots are unreadable. Click them in the legend to bring them back.

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Record low levels and largest anomalies

Calculated over the whole series, 1 January 1960 to 2026-09-06 (24,356 days), combining the BfG record with the live Bonn Pegel readings. All values are on the current gauge zero, so the years before November 1979 have been corrected upwards by 1 metre.

The 20 lowest daily water levels (up to 01 sept 2026)

The 20 most negative anomalies (up to 01 sept 2026)

Interpretation of 'river drought' 

• August 2026 record in absolute terms. Nine days in a row, from 11 to 19 August, sit below every previous reading in 66 years, with a low of 63 cm on 17-18 August. This was a sustained event, not a single spike.
• However, this is not the largest negative departure from normal. Summer levels are naturally low: the normal level in mid-August is about 250 cm, so even 63 cm gives an anomaly of about -189 cm. Winter levels are normally about 385 cm, so January 2017 at 117 cm gives -266 cm. The seasonal cycle limits how negative a summer anomaly can be: for instance, the river cannot fall 260 cm below normal when normal is only 250 cm. The largest anomalies are all winter events, in December-March, concentrated in three episodes: January 2017 (11 days), March 1963 (6 days) and late December 2016 (3 days).
• Comparing against the seasonal 1st percentile: The 2026 days sit around 120 cm below a P01 of about 120 cm for the time of year, while the 2018 days sit near a P01 of about 84 cm. Relative to what is normal for that point in the calendar, 2026 is the most unusual event!