Even in fine weather
- A water intake may suddenly close (obstruction, technical failure, saturation due to excessive water input) : the water – normally channelled into a tunnel – brutally resumes its former course in the river bed.
- A lake that is almost full may overflow if a glacier pocket breaks upstream or if there is an unscheduled stop of the pumping or turbine station fed by the lake.
In case of precipitation
- A water intake purge may start and brutally increase the volume of streams already swollen by rains.
- Also, following droughts, objects may hinder water outflow and form natural dams that, if they suddenly collapse, abruptly release a large water mass into the river bed.
When precipitation ceases
- The basins and water reservoirs are often choked with gravel, branches etc.: a water intake purge may then start causing a sudden rise in the water level downstream.