Winterbourne streams on our chalk soil traditionally run after seasonal rains have percolated through the ground (which holds the water underground as a sponge-like aquifer). As the water table rises, the water starts to flow steadily through fissures in the chalk which we know as springs. This rather wonderful system therefore acts as a reservoir, filtration system and pump all rolled into one.

Flowing and not flowing

In East Garston, we are about five miles from the source of the River Lambourn in Upper Lambourn. This upper part of the river often dries up completely around July (although some years, such as 1976, it doesn’t flow at all; in others, such as 1981, it does so continuously) and returns in the new year. In 2025-26 the river broadly followed this pattern, dying back in mid summer and returning in early January.

From Maiden Court Farm, between East Garston and Great Shefford, the river becomes perennial and flows continuously.

This article by Charlotte Hitchmough from Action for the River Kennet, a very experienced riparian charity which does much work in the River Lambourn, explains more about the importance of winterbournes and the special significance of these seasonal stretches of our precious and rare chalk streams.

The return of the upper reaches of the river can happen very quickly. In 2017 (as this video shows) and 2018, the river sprang back to life mid-February in just over a week.

Predicting and measuring

There are an increasing number of measurements which are taken and computer modelling which can pull together historical and real-time data so that thre groundwater and thus the river that it feeds can be better understood. One such is the Lambourn Valley Flood Forum dashboard, created by Piers Allison of the Pang Valley Flood Forum. This has shown that, after having fallen during the second half of 2025, since mid-December groundwater levels have started to rise.

“For springs in the upper reaches to flow,” the Lambourn Valley Flood Forum (LVFF) FB post reported on 8 January 2026, “groundwater levels at the Longacre borehole [about two miles north of Lambourn] typically need to exceed approximately 131 mAOD [meters above ordnance datum, the actual elevation of the groundwater level referenced to the mean sea level]. As of 8 January 2026, the level was recorded at 132.4 mAOD with a “normal” status for the river downstream. The water table has risen just enough to trigger the springs in Lynch Wood, Lambourn.”

With more rain forecast over the coming weeks, this trend seems set to continue. River levels can vary considerably –  the photo at the bottom of this post shows the same stretch of river as the top image, taken in early March 2020 after record rainfall the month before. It’s sometimes even higher than that.

Sewage and pollution

The return of the river is one thing: what will concern many residents is whether this will also lead to a return of local sewage problems. The main issue, as most people round here are vividly aware, is that as the groundwater rises and the pressure increases, water is forced in through any cracks in the pipes and risks overloading the sewage system.

In recent years, Thames Water – prompted by local organisations like the above-mentioned LVFF and the Sewage Action Group for the Lambourn Upper Valley (SAGLUV) – has performed various upgrades and repairs. Initial indications are that these have improved matters. However, many have yet to be tested in conditions of prolonged high groundwater and rainfall (which can also enter the foul-water system), of which 2025 was largely free.

Thames Water has also taken the precaution of installing (though not yet switching on) two ATAC filtration units in Lambourn and East Garston, should they be needed. These are not solutions for the problem but mitigations. They can, however, help prevent sewage from backing up into people’s homes and ensure that the worst of the discharges do not end up in the river.

The use of tankers is another mitigation, though for TW a much more expensive one. Both may yet be required in 2026 should the recent repairs to the system prove inadequate to deal with whatever our increasingly unpredictable weather brings.

As the springs start feeding the seasonal bourn part of the river, wildlife quickly takes advantage of the extended watercourse. According to local wildlife author Nicola Chester, ducks fly along watercourses and will quickly spot water starting to flow as the springhead moves upstream. In 2018, two pairs landed within hours of the water starting to flow past our house. On 11 January 2026, about a dozen ducks were seen clustering in one small area of water that had recently formed just downstream of our house (see top image). Others are likely to join them in the coming weeks.

It’s not only the birds that are immediately struck by the arrival of the water: our cats and a friendly muntjac that sometimes visits the garden on the opposite riverbank always look most perplexed when the river returns…