THE GEOLOGICAL FORMATION

To tell the story of the river Tanaro we must go back to two million years, to the end of the Pliocene era.
The Alps and the Apennines had nearly totally emerged and the sea which took up the Po Valley was receding. Debris settled by streams was gathering in its place.
About 100,000 years ago the river Po was running in the southern Piedmont to the west of Langhe, while several streams such as the Bormida, the Orba, the Lemme, the Curone and above all the Scrivia, the most important one, were flowing to the east of Langhe.
At that time there was no trace of the present Tanaro: as a matter of fact it was running to the west of the Langhe  hills and was discharging into the river Po about 90 km upriver, near Carmagnola. Later the course of the Tanaro was diverted.
We do not know exactly the chief cause of the deviation of the river eastwards near Cherasco, nor when it took place for sure; perhaps it happened 80,000 or 20,000 years ago.
According to some researches by Professor Sacco, it was likely to be the phenomenon well-known in geology as river-capture, connected with a strong erosional action in which the Tanaro most probably reshaped the hill of Cherasco because it was captured by the Alessandria Valley that is lower than the plain to the south of Turin.

Situation after the river-capture phenomenon

The drainage system of the Alessandria plain changed: the Scrivia stream and its tributaries were pushed towards Tortona while the Tanaro and the Belbo took their places. The sands and the clays of the Tanaro flooded the Alessandria plain.
Looking at a planimetric map you can clearly see that from Niella Tanaro the river becomes a meandering stream, as usually happens in a plain area, whereas it is straight upriver.
Meanders are formed by the slow flowing of the water and according to the phenomenon called "meander jump", they are destroyed by erosion cycles which are caused by the lowering of the base level (or of the middle areas, by the channel works and by digging out building material from riverbeds).
No doubt, also the slow and progressive subsiding of the Po delta caused the erosion cycles and therefore the lowering of the riverbeds. Such a geological process is aimed at rejuvenating all the watercourses flowing into the Po from valleys towards mountains.
Sometimes the "meander jump" does not happen in a natural way, but in an artificial way caused by man. In the areas surrounding Felizzano and Castello d'Annone, during the Genova-Torino railways construction, two meanders have been removed to avoid many bridge constructions.
In those areas the river acts as if at that point its base level had subsided suddenly: the river upstream will be rejuvenated as the water speed will tend to quicken with the consequent increase of the transporting capacity.
Even the variation in confluence points which has been affected by these phenomena, has caused remarkable changes during that time.

 
Variation of the river Tanaro and the river Bormida

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