Dear researchers
I’ve been discussing with friends whether, in the case of a lake, the inflow and outflow are at the same level above sea level – for example, Lake Constance: the inflow from the Rhine at Hard (Austria) and the outflow at the Rhine Falls in Schaffhausen.
In our discussion, we disregarded the curvature of the Earth and asked ourselves: shouldn’t the water in the lake, in theory, be at a consistently even level, i.e. ‘horizontal’?
I would appreciate your feedback.
Kind regards
Karl-Heinz Hafenbrack
Yes, the lake level is practically at the same height. At the inflow, a so-called ‘water bulge’ can occur during high water, which then evens out towards the outflow. If the outflow channel were to suddenly deepen, this would cause a localised drop in the water surface level towards the outflow. Both during high water and when the outflow deepens, the differences in height are typically in the metre range – the water can drain away rapidly under the influence of gravity.
In contrast, in a truly stagnant body of water, the water literally stands still – it does not flow at all. In flowing water, on the other hand, there is constant movement – exactly the opposite. Incidentally, whether water is flowing or not can be measured quite easily using a small ‘wheel’ that turns – or does not turn – depending on the current.
The fact that the moon can cause the surface to tilt slightly, as we know from the tides at the sea, is simply a natural influence that we have to accept.