The earth is getting warmer and the sea level (the level of water in the sea) is rising. Climate change will make high water and low water more frequent events. Read more about climate change.
Climate change will lead to peak precipitation. This means that heavy rain, snow or hail within a very short time will be more common. The river takes this precipitation to the sea. The more precipitation there is in the river, the higher the water level is!
Climate change also causes longer periods of drought. There are times when water levels in the Rhine are low. This is natural for the river. But climate change means that dry periods with no precipitation will be more common. And they will last longer, which means that the water will be even lower!