Abstract: This paper discusses the results from investigating the factors that led to the formation of a catastrophic flood in June 2019 on the Iya river in the city of Tulun, Irkutsk oblast, as well as from calculating and assessing the characteristics of these floods. An analysis of the meteorological environments of flood formation is performed by using the ICON global weather forecast model. The trends in hydrometeorological characteristics for the period 1966–2019 are analyzed. The study revealed an increase in the June precipitation amount by 36–61% (31–46 mm), and negative trends in the maximum water discharges making up 24–37% from May to September. Using data on the water level and historical Q(H) curves, the maximum daily water discharge on June 29, 2019 was estimated at 6800 ± 700 m³/s. A schematization of the Iya river basin is carried out, the parameters are systematized and the distributed Hydrograph hydrological model is verified. For three subbasins within the Iya river basin, the median value of the Nash–Sutcliff criterion was 0.69 for the period 1970–1996. The flood characteristics are simulated and assessed using two sources of meteorological data. According to data from weather stations, the maximum daily discharge was 6570 m³s. This value falls within the range of estimates of observed values but its formation is delayed by 1 day. According to the simulation based on the ICON model data, the maximum 3-hour discharge reached 5260 m³/s, or at least 840 m³/s lower than the observed value; however, the simulated flood hydrograph coincides with observed one for the time intervals. The results of the study confirm a possibility for a short-term flood forecast using the method of deterministic hydrological modeling.