This picture whose I do not know the authorship is going around the web to support the idea (Which I also support) that streams should be left as natural as possible to avoid flooding. But is this image really hydrologically sound?
Let say that the top row is highly simplified and objects size incorrect. In the first top figure, it is unconceivable that vegetation stays at tens or hundreds of meters from the stream. Usually vegetation goes very close to the stream, at least the vegetation that can have its roots inside the water table. This also means that, in figure 2, top row, when the flood comes, it will inundate part of the forest. Finally figure 3 top row should have more evident traces of the flood, for instances some sediment here and there. Correctly, streams are not channels, as the first figure on bottom row wants to convey, where humans make what it should not be done. The human in the picture, however, is not represented at a size compatible with the trees dimensions on top row. From passing for figure on top to the bottom ones, there is a zooming in or the digger should be something like 40 m high if proportion were right. The same for the houses in figure 2, bottom row: they are certainly not proportionate with respect to the trees dimensions. Let’s say that the picture is made to convey sense of disproportion and emphasize that human intervention is wrong. Art freedom, maybe, where the concept is more realistic than the real.
Certainly the rectification of the river is a bad thing and the houses were built too close to the river. Streams are source of ecosystem services and boost the economy so, in the past, it was taken the risk to build close, or even over the river. This was done many times, in a trade-off that was deemed reasonable decades ago, but that it does not seems justifiable anymore now. We bear the weight of history here.
As many knows, human activities alter infiltration in soils and rainfall, consequently, rainfall is more efficiently transformed in runoff. However, unless we claim an extraordinary intervention of climate, the flood in the last figure is grossly exaggerated, if the cause is the city itself. If we think the houses are no more than 15 m (not 40 m as could be suggested by the the proportionality between the top and the bottom rows), the last picture show a ten or more meters of water over the usual level. By far, this cannot be generated (all over and upstream the city until the horizon) by the changes happened in urban soil use. The flood, in fact, is instead reasonably caused by a massive runoff generated in the upstream basin, not by the local urban runoff.
To sum up, the message is shareable "Please give room to river to expand" and "Do not build too close to river" (this increases exposition and vulnerability). However the cause for the flood in the last picture is not the city itself. Certainly, the inappropriate urban planning increased the risk and the losses but it cannot have enhanced the flood that much.
There are obviously cases in which the floods are caused by the city impermeability: this is when the catchment is smaller or of a size comparable to city itself. Just in this case, the soil modifications cause the runoff which in turn causes the flood. The city of Genova, in Italy, is one of these cases in which "urban rivers" exist: but there are many others in our contemporary world where we have many huge megacities (but this it is not the case of this figure).
Concluding, the message is right but the hydrology is wrongly represented. Yes, I know: I have been too picky.
P.S. - Besides water in floods is rarely blue though. Usually, it is full of sediment and of the color of the soil it transports.
