Temperature Measurement
A thermister or thermometer should be used. The probe is held in the
river current, until the reading stabilises. (NB It is important that
it is calibrated correctly)
Influences
Aspect is very important in the Alps. South and south west-facing catchments
receive more insolation and therefore should produce warmer streams.
Altitude is also important. Streams which drain from lower altitudes
should be warmer than those from higher altitudes.
Suspended Sediment Measurement
The simplest way to measure suspended sediment is with the use of a
transparency or turbidity tube. A cross is marked on the bottom of a
transparent tube which is marked in centimetres from the base up its
side. The tube is slowly filled with river water until the cross is
no longer visible when viewed through the water column. The depth of
water at this point is recorded. The greater the depth, the cleaner
the water.
Influences
In mountain steams there are four sources of water, groundwater, surface
runoff snow melt and glacier melt. The turbidity of groundwater is normally
zero since the water is derived from underground sources where there
is no soil to erode. The turbidity of surface runoff will depend on
the land use of the catchment. For example, areas of ploughed fields,
footpath or ski erosion or deforestation may be affected by soil erosion
and thus the water will be turbid. Snow melt usually yields clean water,
but glacial meltwater is often a milky colour because it carries very
fine rock flour produced by glacial abrasion, it is therefore often
turbid. Obviously the further you sample away from a spring, glacier
or snow patch the less its influence.
pH Measurement
pH is a measure of acidity and is measured using a portable hand held
pH meter.
Influences
Natural precipitation is usually slightly acid since rainwater is a
mixture of H20 and C02, producing a weak carbonic acid. (in recent years
the pH of snow in the Alps has been higher. We do not know why.)
When natural water comes into contact with limestone it dissolves the
limestone and thus its pH increases i.e. it changes from an acid (pH
<7) to an alkali (pH>7).
When water comes into contact with leaves. leaf litter and tree trunks
it may also change its pH. For example, coniferous needles and bark
tends to be acid and so produce acid water. Some other tree litter is
alkaline and increases the pH of soil water.
Total Dissolved Solids Measurement
The total dissolved solids (TDS) in a river are measured using portable
conductivity meters. Most meters measure to the nearest 10 mgl-¹
or 1 mgl-¹. TDS is the total of all the dissolved minerals carried
in solution in the river and is measured in milligrammes per litre (mgl-¹)
or parts per million (ppm). The TDS does not differentiate between the
different types of minerals but measures them all, however the majority
are calcium, magnesium, sodium and potassium.
Influences
Precipitation (rain water and snow) is normally fairly pure and contains
few chemicals in solution. Once it reaches vegetation. soil and bedrock
the water begins to dissolve minerals by processes such as chemical
weathering and leaching. This water eventually enters the rivers.
The main influences on TDS are pollution and geology. Generally, calcareous
rocks such as chalks, limestones and dolomites yield high TDS and acid
igneous rocks such as granite and slates yield low TDS.
Bedload Lithology (Geology) Measurement
It is difficult for non-geologists to identify different geologies.
Some understanding of the geology of the bedload is most simply measured
in terms of its calcareous nature. This may be determined by applying
dilute hydrochloric acid to each pebble. Dry a sample of 25 pebbles
and apply a drop of acid. If the bedload pebble effervesces (fizzes)
it is safe to say that it is calcareous. The percentage of calcareous
pebbles in the river at any point can then be recorded.
Influences
The greater the percentage of calcareous rocks in the river catchment,
the greater the percentage of 'fizzers ' which are found in the river
as bedload. Calcareous rocks are limestones, chalks, dolomites plus
any other rocks with significant amounts of calcium in them.
