Olona (Insubrian variants, Ulona, Urona or Uòna) is a river of 131 km long and 1,038 sq km of the basin where the course is developed entirely in Lombardy. The river was born in Rasa, a little village near Varese in Campo dei Fiori, more precisely in the furnaces of Riana. After having crossed the homonymous valley, and the high Po Valley, the River Olona is divided into channels, then turning into a single one, before Legnano. After having passed the town of Parabiago (more precisely through the hamlet of San Lorenzo), the river reaches Rho, where in the case of floods it feeds the floodway channel of the North West. From this point on, the river no longer runs through the natural channel, but an artificial channel known as the "Naviglio Vetra", which is assumed to have been dug by the ancient Romans. Entered in Pero, after an initial still in the open air, the river begins to flow under the road surface. Immediately after wet Milan, gives rise to the southern Lambro. Flows into the Po 'in San Zeno, after receiving abundant water from irrigation canals and fountains. Falls of Valganna  are originated from the river, also known for the same name caves.
It has been one of the most polluted rivers in Italy.