The real-time water quality monitoring network of Evrotas River has been operational in Laconia since July 2004. The experience of more than a year of function has shown that dissolved oxygen is one of the most useful parameters monitored for warning and quality status classification. Concentration of dissolved oxygen in surface water is known to be a critical parameter for water quality and survival of organisms in it. At the same time, it provides an indication of the organic load in water, as well as the potential eutrophication. Dissolved oxygen in surface water normally depends on the atmosphere pressure and temperature that affect oxygen solubility. Real-time monitoring provides a very useful option through data processing that could not otherwise be implemented by conventional monitoring programs of any frequency. It is possible to monitor all daily variations of dissolved oxygen, replacing the unique value of one measurement in a conventional program, with a series of 144 measurements (every 10 min) regressing around a moving average value. This form of time series permits monitoring of the daily amplitude of dissolved oxygen fluctuation, which is an accessory indicator of the water quality status. This frequency permits dissolved oxygen to be utilized as a warning parameter, especially when industrial waste and sewage enters the river system. DO time series can be characteristic of surface water status quality in correspondence to the time series of water temperature. The graph of dissolved oxygen vs. temperature is proposed as a representative graph for overview of surface water quality status. The shapes of the plot concentrations are indicative of different conditions, and useful observations can be made at a glance.