Hydropower impact on the ice jam formation on the upper Bistrita River, Romania

In this work, we investigate the causes of ice jams along the Bistrita River, which has the longest mountainous course (216 km) of any river in Romania. Over a length of 25–30 km on the upstream portion of the Izvoru Muntelui Reservoir, ice block accumulations known as ice jams form almost yearly during the cold season. Analysis of the hydroclimatic and morphological conditions of the riverbed has revealed that a certain combination of their temporal variations is favourable to ice jam formation. The hydraulic geometry of the Bistrita river bed is favourable to the flow of frazil slush, frazil pans, and ice floes while the air temperature is below − 7 °C and the water level of the Izvoru Muntelui Reservoir is below 500 m. Above this level, ice jams block the river bed, and this blockage advances upstream at velocities of several hundred meters per day. The most dramatic instances of this phenomenon were recorded during the winter of 2002–2003, when the thickness of the ice was on the order of 6 m and the resulting floods caused damages and claimed human lives. The appearance in 2003 of the Topoliceni Reservoir, 4 km upstream of the Izvoru Muntelui Reservoir, has complicated the evolution of these winter phenomena, with the lake itself acting as an accumulation pool for ice from upstream. This development has led to damages and inconveniences in canals and at the entrances to power plants, spillways, outlet works, and other hydraulic structures.