River Monitoring Stations in Greater London

The last 18 years average 48.6%, against 52.9% for the first 18 — 8.2% lower.

El Niño winter La Niña winter. Marked for context, not as a cause — these are states of the Pacific, and one wet fortnight moves a British year more than a whole season's pattern does. What El Niño and La Niña are, and what they did here.

The raw count went from 21 to 35 gauges with a high day between 1990 and 2025, while the gauges reporting went from 24 to 104 — which is what the chart divides by.

Each gauge's high level was last re-fitted on 2 October 2026, over that gauge's whole record. It is re-fitted monthly, so these percentages can move a little between rebuilds even in a week with no rain.

What this chart finds is BUNCHING — years when many gauges reached their own high level at once. It cannot tell you whether rivers run higher than they used to, and here is why: each gauge is measured against the level it itself reaches about one day a year over its own whole record, so every gauge crosses that bar about once a year by construction, however wet or dry its river has become. The bar is re-fitted every month as records grow, so a wet decade raises it and damps the very rise you are looking for. Dividing by the gauges that reported keeps the growing fleet out of the count; the like-for-like chart below holds the gauges themselves fixed as well.

The last 9 years average 6.23 per area, against 3.13 per area for the first 9 — 99% higher.

El Niño winter La Niña winter. Marked for context, not as a cause — these are states of the Pacific, and one wet fortnight moves a British year more than a whole season's pattern does. What El Niño and La Niña are, and what they did here.

The raw count went from 1 to 134 alerts and warnings between 2007 and 2025, while the areas warned went from 1 to 28 — which is what the chart divides by.

Raw warning counts have risen steeply since 2006, but so has the number of flood areas that can be warned at all. This chart divides by the areas actually warned, which is the part that is about the weather rather than about the register. Warning policy also changed over the period.

2021: 20.6 spills. 2025: 6.4 spills — 69% lower. That is two years compared, not a trend: the years in between jump about more than the difference between these two.

The raw count went from 1,961 to 1,388 spills between 2021 and 2025, while the monitored overflows went from 95 to 216 — which is what the chart divides by.

Only five years exist, so this is a comparison and not a trend. The number of monitored overflows grew across them, which is why the figure is per overflow. Spills also track the weather: 2022 was a drought summer, and it is one of only five years here, so it pulls the comparison about on its own.