River Monitoring Stations in Dorset

The last 18 years average 36.8%, against 22.9% for the first 18 — 61% 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 30 gauges with a high day between 1990 and 2025, while the gauges reporting went from 4 to 55 — 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.

2021: 21.3 spills. 2025: 20.6 spills — 3.2% lower. Barely different — and with this few years, the ones in between move it more than any direction does.

The raw count went from 4,172 to 4,700 spills between 2021 and 2025, while the monitored overflows went from 196 to 228 — 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.