During the last five years, research on the relation between pollution loads and ecological river water quality has been done on the Dender river. In addition to biological sampling of macroinvertebrates and fish, automated measurement stations were used too to investigate the spatio-temporal variability of physical-chemical water pollution. This study on on-line water quality data collection is based on a measurement campaign during March-April 2000 with two automated measurement stations at two different sites: the flow control weirs at Geraardsbergen and Denderleeuw. These measurement stations contain sensors for temperature, turbidity, conductivity, pH, redoxpotential and dissolved oxygen. Short wave radiation as well as rainfall were monitored by means of pyranometers and rain gauges. A refrigerated sampler with 24 bottles allowed to take samples for additional laboratory analyses. In this study, continuous measurements of two physical-chemical parameters, conductivity and dissolved oxygen, were analysed to evaluate the adequacy of the current monitoring frequency in Flanders. Analysis showed that discrete conductivity measurements can be sufficient for trend detection, but the measuring frequency must be highly increased from one measurement per month to at least 8 measurements. Continuous measurements for conductivity are preferred because extreme values are obtained as well. For dissolved oxygen, a single measurement per month in not enough. The percentage of dissolved oxygen showed a strong diurnal variation with maxima in the late afternoon (photosynthesis) and minima at night (respiration). This parameter also differed significantly from day to day. Continuous measurements are therefore necessary for a reliable assessment of the dissolved oxygen budget of surface waters. When using discrete measurements for dissolved oxygen, a set time should be introduced to eliminate diurnal variation.