In 1999 / 2000 the Tay Foundation supported 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.
|
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).
|
|
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.
|
|
