Based on data from monthly hydrological surveys (1994–2015) and on the historical example of an extreme surge in the mouth area of the Pregolya River (tidal dynamics is absent), the principles of spatio-temporal monitoring of the formation of physical barrier zone are discussed.The approach to the analysis of hydrological conditions in the mixing zone of sea and bay waters based on vertical CTD soundings of the water column through a network of spatially fixed monitoring stations has been expanded by conducting long-term series (from September 10, 2019 to March 27, 2020 – just over 6 months, from October 21, 2022 to December 27, 2023 – one year and two months) of daily vertical soundings at the “control point” in the middle of the mixing zone, which made it possible to identify changes within the synoptic scale. The presence of short-term variability in hydrological characteristics (temperature and salinity) in the barrier mixing zone of the Pregolya River showed the need to use automated measurement methods to record the response of hydrological conditions to storm events. An assessment of the error in determining salinity and temperature associated with the natural spottiness of water and fluctuations in the density jump layer showed that measurements of salinity and temperature should be analyzed with an accuracy of tenths and hundredths, respectively. It was determined that vertical convective mixing during winter cooling cannot occur inside the mouth mixing zone, because this is prevented by the presence of heavier brackish waters.