Source: \Dropbox\web\raspberry_pi\temperature_analysis\

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Water Temperature Measurements - Exeter Quay to Exmouth - 2021-06-29

Sample Measurements

A one way, tide assisted trip from Exeter Quay down the Canal, a portage to the River Exe below St James' Weir and down to Exmouth with a break at Topsham.

Temperature measurements were made from two kayaks.

Trip Timings

  A     09:19 UTC     leave Exeter Quay on canal  
  B     09:35 UTC     leave Canal by Salmonpool Bridge; portage to River  
  C     09:49 UTC     enter River Exe opposite St James' Weir  
  -     10:00 UTC     High Tide Topsham  
  D     10:42 UTC     arrive Topsham  
  E     11:28 UTC     leave Topsham  
  F     12:40 UTC     arrive Exmouth  

Time Series

The graph below shows a time series of temperature measurements (vertical axis) from both kayaks covering the whole period of the trip (horizontal axis). The gaps in the graph correspond to times when the kayaks were out of the water.

    Click image below for full size graph

Starting at the Quay [A] the canal temperature was 17.2°C. Moving down the canal the temperature increased slowly to 18.6°C at Salmonpool Bridge [B].

In the River below St James' Weir [C] the temperature was 15.9°C, and it increased slowly to around 16.7°C at Topsham. After leaving Topsham [E] it continued to slowly increase to 17.4°C (opposite Starcross). Here it dropped sharply to 14.9°C, before recovering slightly to 15.6°C approaching Exmouth [F].


By combining these temperature-time measurements with position-time information from a GPS the location of each temperature measurement can be determined. These values can then be plotted on an interactive map.

In the maps below measurements from both kayaks/sensors are presented as colour coded circles with whites and greens being cold, and reds and purples being warm. By clicking on a circle the actual measurements can be examined.


Lock between Canal and River Exe

The water temperature in the largely isolated canal was 1-2°C warmer than the adjacent river.

Paddling down the canal from the basin to Salmonpool Bridge the temperature rose from 17.2°C to 18.6°C.

However alongside the open lock between the canal and river the water temperature dipped suddenly from 17.5°C to 16.9°C.

This was presumably due to a small amount of cooler water flowing from the river into the canal.

Peak: Riverside Valley Park

Opposite the Riverside Valley Park both kayaks measured a peak in their temperature measurements. The blue kayak, alongside the northern bank, measured a 0.6°C peak, whilst the orange kayak, alonside the southern bank, measured a smaller 0.3°C peak.

On the northern bank there are several small creeks and streams. It is possbile they were feeding sun-warmed water into the river.

Peak: Sewage Works

Both kayaks measured peaks opposite the Countess Weir Sewage Treatment Works. The blue kayak, which was near the southern bank where the works are sited, observed a larger peak.

Peak: Topsham Reed Beds

The two kayaks took different routes around the reed beds. The blue kayak stayed in the main river and the orange one followed the most westerly channel through the reed beds adjacent to the canal.

The temperature profiles were similar until leaving the reed beds where the blue kayak saw an 0.4°C peak.

At this point it was 10:35 UTC, and high tide had been at 10:00 UTC. It is possible that the tide had earlier risen over warm mud flats - however this effect was not apparent at any other points in this area.

Estuary Cooling

Paddling south from Topsham the water temperature steadily increased from 16.6°C to 17.3°C approaching Starcross. Here it suddenly dropped 2.4°C to 14.9°C.

It is possible that this was the boundary between colder sea water which had entered the estuary with the previous high tide and the warmer river water coming down stream.

The trip was undertaken about 4 days after a Spring tide. The tide on the 29th had a tidal coefficient of 81 (this means that the range of the tide on this day was 81% of that of the average Spring tide).

Given that when the tide is out, the Estuary is very shallow, I would have expected that sea water would travel some considerable distance upstream. Maybe taking salinity or density measurements might help to delineate the boundary on another occasion.