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You'll be notified when this gauge is rising more than 0.10 mASD/hr — a sign of rapidly increasing water levels.
You'll be notified when a new storm-overflow (sewage) discharge starts within ~3 km of this gauge, based on the water companies' near-real-time feeds.
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EA/SEPA data and are best-efforts. Terms
Current Level
-0.230 mASD
▬ Steady
24h Max
-0.229 mASD
24h Min
-0.230 mASD
Typical Range
0.01 - 0.94
mASD
Highest Recorded
2.01 mASD
30 May 1992
Since Jan 1987
🚽Sewage overflows nearby
Recent Water Levels
🌍 floodradar.co.uk
1 day
Flood predictions aren't generated for
lock / structure gauges —
their levels are driven by gate operation,
not by catchment rainfall, so the forecast models would have no skill here.
River Flow
90 days
Daily mean discharge (m³/s) with the daily min–max band, from the EA Hydrology API. Flow is the volume of water passing the gauge — often a more direct flood signal than level.
Downstream level
7 days
The level on the DOWNSTREAM side of this lock/weir structure — a separate EA
measure from the main (upstream) reading shown on the Current tab. Threshold bands
are computed from this gauge's own downstream history.
Long Term
🌍 floodradar.co.uk
—
Consecutive rainy days0 → 10+
Days Above Normal
This year vs 5-year monthly range
Recent Readings
Steady over 6h
23 Aug 23:00▬ steady-0.230 mASD
23 Aug 22:45▬ steady-0.230 mASD
23 Aug 21:30▬ steady-0.230 mASD
23 Aug 21:15▬ steady-0.230 mASD
23 Aug 21:00▬ steady-0.230 mASD
23 Aug 20:45▬ steady-0.230 mASD
23 Aug 20:30▬ steady-0.230 mASD
23 Aug 20:15▬ steady-0.230 mASD
23 Aug 20:00▬ steady-0.230 mASD
23 Aug 19:45▬ steady-0.230 mASD
23 Aug 19:30▬ steady-0.230 mASD
23 Aug 19:15▬ steady-0.230 mASD
23 Aug 19:00▬ steady-0.230 mASD
23 Aug 18:45▬ steady-0.230 mASD
23 Aug 18:30▬ steady-0.230 mASD
23 Aug 18:15▬ steady-0.230 mASD
23 Aug 18:00▼ -0.1 cm-0.230 mASD
23 Aug 17:45▲ +0.1 cm-0.229 mASD
23 Aug 17:30▬ steady-0.230 mASD
23 Aug 17:15-0.230 mASD
Rainfall & Weather
Loading rainfall and weather data...
Flood history near this gauge
Official Environment Agency warnings issued since 2011 for the 3 flood
areas linked to this gauge (nearest 8.0 km away).
Warnings apply to a whole flood area, not necessarily this exact spot. Years before 2006 aren't covered by the EA's historic record.
See also the Grendon Underwood flood history.
Geographic coordinates may reflect the access point for the gauge, and not the precise location in the watercourse that it is taking readings from.
Catchment Character
🌊 FlashyRain runs off fast — levels rise and fall sharply, often within hours of heavy rain.
Your catchment is all the land that drains rainfall into the river above this gauge. Its character — how
steep, wet, wooded, urban or chalky that land is — decides how fast rain here turns into a rising river. A steep, bare or clay
catchment is “flashy” (rises and falls quickly, floods easily); a flat, chalk or well-wooded one is
slow and steady. The tiles below put numbers to that; the flow-duration curve reads left-to-right
from the river’s typical high flows (Q5) down to its low flows (Q95) — a steep curve means a flashy river.
The physical character of the land that drains to this gauge, and how it shapes its flood response — from the National River Flow Archive (UKCEH).
Baseflow Index
0.24
flashy — rain runs off fast
Avg Annual Rainfall
655 mm/yr
1991–2020 · +1% since 1941–70
Urban Extent
0.4%
largely rural
Highest Recorded Flow
5.80 m³/s
3 Jun 2008
Flow-Duration Curve
Daily flow (m³/s) exceeded each % of the time — a steep curve is flashy, a flat one is steady. Q5 high · Q50 median · Q95 low.
Representative catchment that is important in terms of a national network. It is now a key gauging station for climate change research in the UK. Operated as an experimental basin by IH until 1987. Subsequently managed by Thames Water and then by the EA.
More detail
The catchment
Relatively flat, impermeable (Oxford Clay) catchment largely given over to agriculture.
The station
Flat V weir, rebuilt in 1998, within a channel 6.5m wide. The first records for the new weir from 26 March, 1999. Originally a trapezoidal critical-depth flume with the capacity increased in 1964 to make station full range.
How it measures flow
Easily drowns owing to restriction caused by d/s culvert. Weir likely to become non-modular before bankful limit. Bypassing observed in exceptional floods but is not rated. Point at which the weir drowned (and therefore the flow calculation is completely uncertain) depends on d/s water levels, may be between 0.3m and 0.55m u/s stage (equivalent to approx 1.5 cumecs). One rating applied across period of record that operates to a standard theoretical rating. The weir is non modular during peak flows as the structure is designed primarily as a low flow structure. The last rating review was 2013. The modular limit is less than Qmed.
Flow regime
Negligible artificial disturbance to the very flashy, responsive flow regime.
The flow record
Some data loss during 1998 rebuild. Data from 1982-86 less reliable and incomplete. Natural flows to within 10% at Q95.
How this gauging station is built and how long it has measured (NRFA / UKCEH).
Gauge type
Flat-V weir
Operating since
1962
Station datum
65.7 mAOD
Flow record
1962–2024 · 62 yrs · 95% complete
Hydrometric area
39
Measured by
Environment Agency
Nearby Groundwater
Soil Moisture
🚽 Storm overflows on this river
Frequently asked questions
Is the River Ray at Grendon Underwood flooding today?
The current level is -0.23m, which is below the typical range. Flooding is very unlikely at this level.
Is the river level rising or falling at Grendon Underwood?
Based on the most recent readings, the level is currently steady. The chart above shows the last 7 days of measurements, updated roughly every 15 minutes.
What is a normal level for the River Ray at Grendon Underwood?
The typical range at this station is 0.01m to 0.94m, based on Environment Agency historical data. Levels above 0.94m are higher than usually seen and may indicate a flood risk.
How often is this river level data updated?
Readings are taken by the Environment Agency gauge roughly every 15 minutes and FloodRadar refreshes its data continuously, so the level shown is normally less than an hour old.
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Free live river-level widget — ideal for parish councils, angling clubs and local sites. Updates automatically.
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📮 Flood risk by postcode near this gauge
Postcodes within 5 km with properties in the Environment Agency’s rivers-and-sea flood-risk bands:
