...
The 1963 P01 values look odd next to 2017's — around 198 cm for early March against ~124 cm for early January. That isn't an error: the percentiles come from the 1990–2021 climatology, which has a much higher 1st percentile in March than in January. It does mean the 1963 days sit far below their seasonal P01 while several 2017 days sit just above theirs, which is why the "below P01" column disagrees with the anomaly ranking in places.
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
| # | Date | Level (cm) | Anomaly (cm) | Normal for the date (cm) | P01 (cm) | Source |
|---|---|---|---|---|---|---|
| 1 | 2026-08-17 | 63 | -188.6 | 251 | 120 | live |
| 2 | 2026-08-18 | 63 | -186.3 | 249 | 119 | live |
| 3 | 2026-08-16 | 64 | -188.6 | 253 | 121 | live |
| 4 | 2026-08-15 | 64 | -189.6 | 254 | 122 | live |
| 5 | 2026-08-13 | 66 | -188.1 | 254 | 123 | live |
| 6 | 2026-08-14 | 67 | -187.6 | 255 | 123 | live |
| 7 | 2026-08-12 | 70 | -184.6 | 254 | 123 | live |
| 8 | 2026-08-19 | 70 | -177.4 | 247 | 118 | live |
| 9 | 2026-08-11 | 71 | -183.0 | 254 | 123 | live |
| 10 | 2026-08-07 | 75 | -178.2 | 253 | 126 | BfG |
| 11 | 2026-08-10 | 75 | -178.4 | 253 | 124 | BfG |
| 12 | 2026-08-06 | 77 | -177.2 | 254 | 126 | BfG |
| 13 | 2026-08-08 | 77 | -175.8 | 253 | 126 | BfG |
| 14 | 2026-08-09 | 78 | -174.9 | 253 | 125 | BfG |
| 15 | 2026-08-20 | 79 | -166.4 | 245 | 116 | live |
| 16 | 2026-08-01 | 80 | -183.7 | 264 | 132 | BfG |
| 17 | 2018-10-22 | 81 | -144.1 | 225 | 84 | BfG |
| 18 | 2018-10-23 | 81 | -145.2 | 226 | 84 | BfG |
| 19 | 2026-08-03 | 81 | -178.0 | 259 | 128 | BfG |
| 20 | 2018-10-25 | 82 | -147.1 | 229 | 83 | BfG |
The 20 most negative anomalies
| # | Date | Level (cm) | Anomaly (cm) | Normal for the date (cm) | P01 (cm) | Source |
|---|---|---|---|---|---|---|
| 1 | 2017-01-10 | 117 | -266.5 | 383 | 124 | BfG |
| 2 | 2017-01-09 | 118 | -266.2 | 384 | 124 | BfG |
| 3 | 2017-01-08 | 120 | -263.9 | 384 | 123 | BfG |
| 4 | 2017-01-03 | 123 | -260.9 | 384 | 125 | BfG |
| 5 | 2017-01-04 | 124 | -260.3 | 384 | 124 | BfG |
| 6 | 1963-03-04 | 116 | -260.2 | 376 | 198 | BfG |
| 7 | 2017-01-02 | 124 | -259.4 | 383 | 125 | BfG |
| 8 | 1963-03-05 | 116 | -259.0 | 375 | 197 | BfG |
| 9 | 2017-01-11 | 123 | -258.8 | 382 | 124 | BfG |
| 10 | 2017-01-01 | 125 | -258.6 | 384 | 126 | BfG |
| 11 | 2017-01-05 | 126 | -258.5 | 384 | 124 | BfG |
| 12 | 2017-01-07 | 125 | -258.4 | 383 | 123 | BfG |
| 13 | 1963-03-03 | 118 | -258.3 | 376 | 198 | BfG |
| 14 | 1963-03-02 | 119 | -256.3 | 375 | 201 | BfG |
| 15 | 2016-12-31 | 129 | -255.6 | 385 | 126 | BfG |
| 16 | 2017-01-06 | 128 | -255.6 | 384 | 123 | BfG |
| 17 | 1963-02-12 | 111 | -254.6 | 366 | 149 | BfG |
| 18 | 1963-03-06 | 121 | -253.8 | 375 | 194 | BfG |
| 19 | 2016-12-29 | 132 | -253.5 | 385 | 131 | BfG |
| 20 | 2016-12-30 | 132 | -253.4 | 385 | 127 | BfG |
What this means
The two tables answer different questions, and 2026 dominates one while being completely absent from the other.
August 2026 is a genuine 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. It also happened after the BfG record ends, so it is only visible thanks to the live gauge readings.
But it is not the largest 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: 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). Six of these are from before November 1979 and depend on the 1-metre datum correction; without it they would appear far more extreme than they really were.
Comparing against the seasonal 1st percentile sharpens the picture. 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 more unusual event.
Table generated automatically on 07 September 2026.