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Water Temperature Measurements - River Exe and Exeter Canal - 2020-09-04


These measurements were taken on a kayak trip from Exeter Quay ...

The river section was undertaken downstream about 2 hours after a high Spring tide.

Time Series

The graph below shows a time series of temperature measurements (vertical axis) covering a five hour period (horizontal axis). The gaps show times when the kayak was out of the water.

    Time Table
  08:51     09:10     Canal Basin to Salmonpool Bridge  
  09:24     10:12     St James' Weir to Topsham Quay  
  11:00     11:10     Topsham Quay to Ferry Steps  
  11:18     12:42     Ferry Cottage to Double Locks  
  13:17     14:07     Double Locks to Canal Basin  

The temperature traces show a series of steep 'spikes'. There is one significant spike in the river section (labelled Q) and at least a dozen in the canal (labelled A-K). The river spike is about 1.7°C high but some of the canal spikes are as large as 4°C.

Measurements Overlaid on Maps

The following two links show measurements of temperature and the amount of weed overlaid on interactive maps. The colour scales used to depict the temperature are different on the river and the canal but the detailed values can be read by clicking on the circles.

River Measurements

The temperature measurements taken in the river were relatively uniform at around 14.7°C. There was however one significant spike to 16.4°C.

The above map shows the location of this temperature spike - adjacent to the Countess Wear Sewage Plant.

Detailed Canal Measurements

The section of canal between Ferry Cottage and Double Locks contained numerous patches of weed.

Some patches were anchored to the bottom and were sufficiently thick to make progress difficult - this was particularly the case for paddle boarders with fixed fins.

The impact of weed was explored using three sets of data:-

The weed observations were made visually the following day, from the bank, using a three point scale as follows - clear, moderate and total coverage.

    Subjective Weed Coverage Scale
Clear Green   <10% weed coverage across width  
Moderate Yellow   10-90% coverage across width  
Total Red   weeds covering full width of canal  

The following graph shows the three sets of 'measurements' plotted as a function of distance (in metres) from Ferry Cottage. The vertical scale shows water temperature in °C and relates to the orange line.

The green line indicates the quantity of weed on the above three point subjective scale. The lowest points indicate no weed and the highest points total weed coverage across the canal.

The blue line indicates the kayak speed. The lowest point on the blue line (at about 350m) indicates 0 m/s and the highest point (at about 2200 m) indicates 1.99 m/s (4.55 mph)

It would appear that most of the high temperature spikes in the canal are associated with regions of significant weed coverage. These clearly have an impact on the speed of the kayak.

It is not immediately clear whether

The above graph shows schematically the vertical temperature of the sea in daytime light wind conditions and in night time (or strong wind) conditions.

In strong winds the water column near the surface is mixed so that the temperature is uniform.

In daytime light wind conditions solar heating warms the top few centimetres of the water. This effect is of considerable interest to scientists making sea surface temperature measurements from satellites.

It seems likely that the weed restricts convective and wind driven currents which would mix warmer surface water with cooler deeper water.


Towing a paddle-boarder out of the weeds

Mapping Temperature and Weed

The pairs of maps below show the temperature and weed observations.

The first map of each pair shows the temperature measurements. Green circles show 'low' temperatures (~18°C) and yellow and purple circles show 'high' temperatures (20-22°C).

The second map of each pair shows the subjective assessment of the weed coverage in the canal.

Topsham Lock to M5 Bridge

M5 Bridge to Bridge Road

Bridge Road to Double Locks