In 1999 / 2000 the Boad undetook an investigation
into water temperatures in two dammed tributaries of
the River Tay – the River Lyon and the Errochty
Water. Loggers which measured the water temperature
every hour were installed.
Both the Lyon and the Errochty are maintained by
compensation flows which are drawn from the lower
levels of hydro reservoirs, the dams on the Errochty
and the Lyon being the highest in the Tay system.
The study showed that in spring and summers the temperature
of the water close to the dam displayed a relatively
low daily variation and was relatively cold. Natural
temperatures would have been higher as evidenced
by the fact that going progressively downstream temperatures
did warm up because of warming by the sun and the
inflow of natural tributaries. In winter, however,
the dam water was relatively warm.
In the spring the temperature of the water issuing
from the dams tended to rise over time in a stepped
manner. The temperature might remain relatively constant
for days or weeks then suddenly rise. The steady
periods tended to coincide with warm weather and
the jumps with cooler weather. This shows that during
warm weather these reservoirs become stratified,
i.e. warm water basically floats on top of cold water,
but when a depression passes over the waters become
mixed so the bottom water temperature increases.
The unnatural temperature regime below these dams
will have ecological impacts.
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Daily water temperature readings on the Errochty
Water, mid March 2000 to mid June 2001. Dark
green line represents average daily temperature
near the dam at Trinafour (light green line
represents hourly temperature readings) and
the red line represents average daily temperatures
at a location approximately four miles downstream
from Trinafour (orange represents hourly readings).
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Hourly temperature readings at three sites
on the River Lyon April / early May 2000. Lubreoch
Dam is the source of the flow, Kenknock is
approximately six kilometres downstream and
Invervar is approximately 25 kilometres downstream.
The Fillan Water is a neighbouring unregulated
catchment used as a control. Comparing the
Fillan with Invervar, Lyon temperatures have
more of less naturalised by that distance,
but in the upper Lyon there was a significant
impact on temperature at this biologically
important time of year.
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