The base flow is the core flow of the river, the water that is there even when it has not rained. It comes from the groundwater in the chalk hills, which is highest in spring and lowest in autumn. In a dry year the groundwater drops so far that the base flow dies away and the river can even stop flowing at Bucklebury ford.
The run-off is added to the base flow when we have rain. How much is added depends on the rain event: how much rain falls and how wet the ground already is.
The hills in the north are chalk. It soaks up rain like a sponge and lets it out slowly at springs. The Hodcott 2 borehole, up on the chalk, measures how full it is.
No streams join the river until just downstream of Marlston, so the water passing the Frilsham weir gauge is almost all groundwater.
Rain makes the river spike for a day or two, then it falls back to the base flow. We use the lowest reading each week so the spikes do not hide it. Week by week, the river rises and falls with the groundwater (correlation 0.89, where 1 would be perfect).
Data: Environment Agency. A few faulty readings are left out.
Rain takes weeks to soak down into the chalk, then keeps topping it up for most of a year. So the groundwater today is the last 12 months of rain added up, with two adjustments:
That simple sum, from rainfall alone, follows the real groundwater well. Little rain since March means little new water in the chalk, so the groundwater falls and the river falls with it.
Pink: measured groundwater. Blue: worked out from rainfall alone.
South of the river the chalk is covered by clay, in a ridge from Cold Ash through Upper Bucklebury to Bradfield Southend. Clay does not soak rain up, so in wet weather it runs off down the streams into the Pang. That run-off comes on top of the base flow.
It depends on the season. In our records a typical 20mm rain event lifts the Bucklebury gauge by about 13cm in winter, when the ground is wet, but only about 3cm in summer, when it is dry.
Each dot is one rain event and how far the Bucklebury gauge rose afterwards. Blue: October to April. Orange: May to September. Winters 2023/24 and 2024/25 are left out because trees were in the river below the gauge. More detail: Bucklebury NFM.
Briff Lane sewage works. Small amounts of treated water from the Thames Water works run into the Pang down the Briff Lane Stream. It is a steady trickle, so there is a small flow even when Bucklebury ford is dry.
Ingle Spring. Just above Stanford Dingley, it adds a little more water.
The Blue Pools. Below Stanford Dingley and above Bradfield, these permanent springs add a lot more water, so the river below them keeps flowing even in a dry summer.
West Berkshire Groundwater Scheme. In a drought the Environment Agency can pump groundwater out of the chalk and into the river, through the pipes and outfalls on the map, on its way to London's reservoirs. It keeps the river higher for now, but the groundwater falls further. It has been on since late August 2026 (see our map).
River weed. In summer, weed grows thick in the channel and holds the water back. The level at a gauge goes up, even though the amount of water flowing has not changed, so summer and winter levels cannot be compared directly.