Sampling in the framework of national river monitoring networks is usually carried out during low flow season, optimally during the natural low-water period of the respective water body under clear water conditions (i.e. summer in Europe).
- Flood events must be taken into consideration:
If low intensity hydrological events appeared (low intensity floods, floods of a few days duration, aerial exposure of a few days), it is recommended to wait a few days before taking samples. After intense hydrological events (lasting floods, floods causing a reworking of the supports), it is recommended to wait 3-4 weeks before taking the samples.
In the particular case of a sampling station subjected to strong artificial variations of the flow (after dams for instance), the artificial hydrological regime is assimilated to a stabilized flow.Choice of the sampling stations (example of application in Eco-AlpsWater monitoring)
Benthic algae biomonitoring in rivers (e.g. to establish diatom indices as IBD, IPS, TDI) is applicable to stations located on natural or artificialized watercourses with the exception of naturally salted stretches (e.g. estuaries). It is also necessary that the substrates used for sampling have been submerged during enough time (several weeks) and are accessible.
If the sampling station has been previously defined (for instance by an environmental agency, or a water agency), the samples will be taken at the defined place, unless the sampling conditions (representativeness, accessibility ...) lead the person in charge of the sampling to choose another place.
In the framework of EcoAlpsWaters, we recommend to sample on pre-existing stations defined by the environmental agency in charge of the regular monitoring of the river.
If the sampling station is not defined, then a stretch of minimum 20m/40m or 4-5 times the width of the running water body has to be sampled (Federal Ministry of Agriculture, Forestry, Environment and Water Management 2015).