River Temperature Relationships: A 15-year monitoring programme
Our 2025 river temperature loggers are back in, data downloaded, and we’ve been digging into what this summer showed across the Wessex Rivers Trust network.
Since 2011, WessexRT has tracked river temperatures through summer using loggers placed in shaded locations within the main channel flow. Each logger records temperatures every 15 minutes from the start of June to the end of September. Loggers are retrieved each autumn for downloading.
We now monitor nine routine sites across catchments, and in earlier years deployed loggers at more than twenty locations. Together, these measurements provide a long-term view of how rivers respond to weather patterns, river flows, and groundwater conditions.
How river temperature is shaped
River temperature is influenced by:
• Air temperature and sunshine
• River flow and travel time (slow, shallow water heats more)
• Groundwater discharge (cooler inputs buffer warming)
• Position along the river (water warms downstream)
That’s why we compare sites using a simple distance-from-source measure. Across all years, water becomes warmer as it flows downstream, letting us compare equivalent upstream/downstream sites in different catchments.
What the long record tells us (2011–2025)
Two clear patterns emerge:
1. Shared regional weather signal
Across all nine sites, summer river temperatures rise and fall in step with the wider weather pattern. Both the highest daily peaks and the monthly averages respond to the same regional hot or cool spells.
2. Consistent downstream warming
Throughout the series, downstream sites are warmer in summer than headwaters, reflecting accumulated heating as water travels.
Why 2023 and 2024 were cooler summers in rivers
River flows and chalk groundwater levels were relatively high in 2023 and 2024. That meant:
• faster movement through river reaches,
• more cool groundwater discharge, and
• less opportunity for water to heat during hot spells.
In practice, this buffered river temperatures and limited peaks.
Summer 2025: low flows, warm air, and widespread heating
Summer 2025 followed a long cool spring, with only brief wet spells early on. These had little lasting effect on declining discharge. By mid-summer, flows were low for extended periods, while air temperatures were above average.
The results were striking:
Monthly average temperatures
• June 2025: almost every monitoring site recorded one of its three warmest Junes in the entire 2011–2025 record.
• July 2025: every site recorded its warmest or second-warmest July on record.
• August 2025: nearly all sites again sat within their top three warmest Augusts.
• September 2025: temperatures dropped back quickly, giving relatively low values network-wide.
Annual maximum temperatures
• Over the period 2013–2025, two of the nine sites recorded their highest annual maximum temperature in 2025, signalling particularly strong heat-stress potential under low-flow conditions.
Why this matters for river health
River temperature affects oxygen levels, habitat suitability, and stress thresholds for fish and invertebrates. The 2025 summer reinforces a key long-term insight:
Low summer flows amplify river warming and heat-stress risk.
High groundwater and higher flows damp it.
As climate patterns shift and dry summers become more frequent, understanding these relationships helps us identify where rivers are most vulnerable — and where protection or intervention can be most effective.
Next steps
WessexRT will continue summer monitoring across all routine sites and expand coverage where needed. This ongoing record will help us track:
• emerging heat-risk hotspots,
• the role of groundwater buffering, and
• how different catchments respond to future dry, warm summers.
