A record-hot summer arrives: how London's reservoirs are taking it

The first reservoir post stopped on 31 May 2026, with both of London’s big storage groups brimming: the Lower Lee at 95% of capacity, the Lower Thames at 90%. A comfortable place to end a story about water.

Then the summer came. The Met Office now says a record-warm UK summer is increasingly likely, with temperatures staying well above average. So I did the obvious thing: grabbed the June and July readings, added them to the same 37-year daily series, and looked. I wanted to resist the tidy headline (“record heat drains the reservoirs!”) and let the readings talk first, because they usually say something more interesting than the slogan.

Data & licence. London reservoir levels via the London Datastore, sourced from the Environment Agency’s water situation reports. Public sector information licensed under the Open Government Licence v2.0. Data now runs 1 Jan 1989 to 31 Jul 2026. The “warmest summer” framing is context from the Met Office (link above); this analysis works only with reservoir levels, not temperature. Notebook: reservoir_analysis_2026-07.ipynb.

First, just add the days

No cleverness here. I extended the sawtooth by two months and shaded 2026 so the newest readings stand out against four decades.

Every daily reading, 1989-2026; the 2026 window is shaded. Drag to zoom into the tail.

Zoom into the right-hand edge and the shape is clear: after peaking in spring, both lines fall through June and July. The orange Lower Thames line keeps going, sliding to 72% by 31 July (the Lower Lee ends at 82%). That’s a real drop, but a real drop isn’t a finding. The daily view can’t tell me whether 72% is alarming or ordinary for late July. For that I need to compare 2026 against its own history.

How unusual is it? Draw the normal band

The honest test is the same one that flagged 2022 in the first post: plot 2026 on top of the normal band. The grey envelope is the full day-by-day min-to-max of every other year (1989-2025); the dashed line is the typical year. If 2026 sits inside the band, it’s within normal. If it rides the edge, it isn’t.

2026 (coloured) against the 1989-2025 daily range (grey) and the typical year (dashed), for each group.

Here the two groups part ways, and this is the actual story.

The Lower Thames (bottom panel) starts the year mid-band, tracks the typical line into spring, then peels away and rides the bottom edge of the grey band all summer. Its 72% on 31 July isn’t just low: it matches the lowest level ever recorded for that date in the 37-year record. For late July, the Thames group has never been emptier. It’s on the floor of its own range.

The Lower Lee (top panel) tells a softer version. It fell too, but it’s sitting comfortably inside the band, around the 24th percentile for the date: low-ish, nowhere near a record. Same city, same summer, two different responses. The structural gap between these systems, which the first post flagged, is doing real work here.

So the headline the slogan wanted, “record heat empties the reservoirs,” is half right and half wrong, and the interesting half is the wrong one. One group is at a record low for the date; the other is just a bit dry. Which raises the obvious question: did the Thames get there because this summer drained it unusually fast?

Was the drawdown actually the fastest?

A hot, dry summer should show up as speed: how far the reservoirs fall between late spring and mid-summer. So for every year I measured the June-to-July drawdown (average level in the last week of May minus the last week of July) and ranked them.

Lower Thames: the size of the late-May-to-late-July fall, by year. 2026 is in red.

And here the data complicates the neat narrative. The Lower Thames fell about 17 points this summer, steep enough to rank 5th-fastest of 38 summers, but not the record. 2022 fell faster (about 24 points), and so did several other years. The Lower Lee’s 13-point fall ranks 7th; its record belongs to 2018 (about 22 points). A summer the Met Office thinks may be the warmest on record did not produce the fastest drawdown on record.

So how is the Thames at a record low for the date if the fall wasn’t record-breaking? Because of where it started. The Lee refilled to a brimming 100% over winter; the Thames only reached about 93%. A slightly lower starting line, plus a steep but not unprecedented fall, was enough to land it on the floor. The record isn’t about how hard this summer pulled. It’s about the summer landing on a spring that never quite topped the Thames up.

What the data says

Reading the data rather than the headline:

The Lower Thames is at a record low for the date: 72% on 31 July, matching the emptiest late-July in 37 years. That part of the scary story is true.
But the drawdown wasn’t the fastest. 2022 and 2018 both drained faster. Record heat did not mean record-speed emptying.
Starting height mattered as much as the heat. The Thames began summer a touch lower than the Lee and ended on the floor; the Lee started full and stayed inside its normal band.
The real test is still ahead. July isn’t the seasonal trough; that comes in September or October. If the Thames keeps hugging the bottom edge, that autumn minimum is the number to watch, against the deep droughts of 1996-97 the first post dug up.

None of this downplays the summer. A record-low-for-the-date reservoir is worth watching. But the outcome I’d have guessed (“hottest summer, fastest drain”) isn’t the one the data supports. The reservoirs are spending a full winter’s savings fast, not running on empty, and the number that will really matter hasn’t happened yet. I’ll come back in October. The notebook is here if you want to watch the floor with me.