T Koltsova and J Belakova

Latvian Environment, Geology and Meteorology Agency, 165, Maskavas St., LV-1019 Riga, Latvia

Abstract. The Lielupe River is the second largest river in Latvia. Annual water runoff of the Lielupe River to the Gulf of Riga is 3.37 km3. After Jelgava city, the river starts forming a typical estuary with riverbanks, small islands and peninsulas, swamps etc. From here and down to the river mouth, the gradient of the river is 5-10 cm per kilometre, and on occasion the high water level in the Gulf of Riga has a damming effect on the flow of the river. The riverbed is much lower than the average Baltic Sea level over a length of 100 km upstream from the mouth. As a result, the river flows in the opposite direction from the sea in autumn and winter. In low flow periods in summer and winter, the water quality conditions become critical, because of decreasing oxygen content and increasing oxygen demand. The autumn of 2005 was characterised by very low flow and high water temperature. Hydrometeorological conditions plus the wastewater from the Jelgava sugar factory led to the formation of a dissolved oxygen deficit zone and fish losses as a result. Poor water management and water security were the main cause of that disaster. Storm surges facilitate the exchange of the water mass and thereby improve the quality of the water. Air temperature, wind direction and speed and water level data series were analysed with respect to climate change impacts on the water quality of the Lielupe River.

Keywords: Storm surges, backwaters, oxygen deficit zone, wind rose, wind gust index

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