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Water Temperature Measurements - 2020-10-11 - River Fowey

This paddle was a trip from Fowey to Lostwithiel and back. It was timed to take advantage of the in-coming (Neap) tide upstream and the out-going tide downstream.

    Time Table
Start (UTC)End (UTC)Activity
  05:08       Low Water Fowey  
  09:16     11:48     Fowey to Lostwithiel  
  11:37       High Water Fowey  
  12:27     14:05     Lostwithiel to Fowey  
  18:28         Low Water Fowey  

There was a strong northerly breeze - so the outward leg was substantially slower than the return one.

There was also a strong flow of river water. Just below the old bridge in Lostwithiel it was not possible to make progress upstream when paddling at 4.5 mph.

The graph below shows a time series of temperature measurements (vertical axis) covering the 5 hour period of the paddle (horizontal axis).

In the river section below Lostwithiel the water temperature was an almost uniform 11.3°C. In the estuary the temperature reached 13.3°C on the upward leg and 14.4°C on the return leg.

The off shore sea temperature was 15.1°C.

The two maps below show colour coded temperature measurements. Each colour corresponds to a temperature range of 0.3°C.

The left hand map shows the upstream temperatures (Fowey to Lostwithiel) and the right hand map shows the downstream temperatures (Lostwithiel to Fowey).

The same data can be viewed interactively on this map. Clicking on a circle brings up a pop-up which shows the temperature measurement at that point.

The paddle took place during a Neap tide. The relatively fast moving cool river water extended some 3 km downstream from Lostwithiel Bridge (almost as far as St Winnow Church). Downstream of this point it appears to mix with warmer water coming in with the tide. Presumably during a high Spring tide the sea water would reach further towards the tidal limit shown on OS maps as just above Lostwithiel Bridge.

During the return trip the water in the lower estuary was substantially warmer than on the outward trip. It seems likely that the in-coming tide introduced warmer water (warm that is relative to the river water mixed with the previous tide) into the estuary which we then encountered on the return trip.