Olona
The Olona is an Italian river belonging to the Po Basin, long, that runs through the Province of Varese and Metropolitan City of Milan whose course is developed entirely in Lombardy.
The river born at 548 meters above sea level in the Fornaci della Riana locality at the Rasa of Varese, at the Sacro Monte di Varese, within the Campo dei Fiori Regional Park. After crossing the Valle Olona and the Alto Milanese, the Olona reaches Rho where it pours part of its water into the Canale Scolmatore Nord Ovest. After passing Pero, the river enters in Milan, where, at the exit of its underground route, it flows into the Lambro Meridionale, that flows into the Lambro at Sant'Angelo Lodigiano, in the quartier of San Cristoforo ending its course. Along the way, the water system formed by the Olona and the Lambro Meridionale crosses or laps 45 towns receiving the water of 19 tributaries.
The Olona is known for the waterfalls and caves of Valganna and for having been one of the most polluted rivers in Italy. The valley carved by the river, thanks to the system of water wheels that exploited the driving force originated by the water, was one of the cradles of Italian industrialization. The Olona river consortium, that is founded in 1606, is the oldest irrigation consortium in Italy.
The river is sometimes also referred to as "northern Olona" for the homonymy with another Olona, who was born in Bornasco and flows into the Po after having crossed the Province of Pavia. This second Olona, in turn, is designated as "inferior" or "southern". The homonymy is not of imitative or etymological origin, but it is due to the fact that originally it was two trunks of the same river, diverted by the ancient Romans in its upper stretch towards Milan to bring water to the moat of the defensive walls of the city.
Physical geography
Path
After the initial stretch, the river begins to travel the valley of the same name, the Valle Olona. This valley originated from the Olona and the retreat of the glaciers during the last ice age; it looks like a valley deeply engraved with the inhabited centers located on the hills overlooking the river bed, the so-called pianalti.The slopes are mostly covered with woods, while in the valley bottom there are cultivated areas, meadows and heaths. The main tributary on Olona in the province of Varese is the Bevera; other important tributaries of this stretch of the river are the Vellone, the Gaggiolo, the Quadronna, the Selvagna, the Mornaga, the Riale delle Selve and the Tenore.
In Gorla Minore the river branches off into the Olonella, which joins the main riverbed after 1200 m. In this stretch, many artificial canals are born in the service of agriculture and of the industries that re-enter the Olona before Castellanza. After passing Castellanza, the waterway leaves the Valle Olona and heads towards the Po Valley. Once there was also another natural branch which was indicated by the name of Olonella and which crossed Legnano passing behind the basilica of San Magno. The natural island that was formed by the two branches of the river was known as "Archbishop Braida". This legnanese branch was buried in the first part of the 20th century.
After crossing San Lorenzo and Nerviano, the Olona river receives its two main tributaries, the Bozzente and the Lura, at Rho. In Rho there is also the "deviator of the Olona", completed in the 1980s, which flows into Lambro Meridionale. This work has always been a source of controversy: on the one hand it has not avoided all the floods as it was designed and, on the other hand, it carries polluted water towards the Ticino. In November 2002, in particular climatic conditions, the Olona, the Seveso, the spillway and the Ticino overflowed.
From Lucernate onwards the river no longer flows into the natural riverbed, but follows the path deviated by the ancient Romans towards the Bozzente. Entering Pero, after an initial stretch still outdoors, the Olona begins to flow under the road surface and reaches Milan by first crossing the Gallaratese, Lampugnano and QT8 districts, where it collects the water of the Merlata river, to then skirt the southern slope of Monte Stella. Once in Piazza Stuparich it receives the confluence of the Pudiga.
Merlata and Pudiga are the water collectors that come from the area north of Milan, the so-called "groane". The Olona then runs along the Lampugnano and San Siro districts, and then continues under the avenues of the ring road bypass.
The path under the ring road, which was designed for the first time on the General Plan of Milan in 1884, was channeled in the first two decades of the 20th century and covered in a period from 1950 to 1970. At the exit from this covered stretch, after passing under the Naviglio Grande, the Olona ends its course flowing into the Lambro Meridionale.
Until the entry into operation of the water purification system of Milan, the Lambro Meridionale was a drain collector which collected the results of the western part of the city sewerage system. Subsequently, these sewage were diverted to the purifiers of San Rocco and Nosedo. From the same unloader, the Lambro Meridionale has its "clean source".
Basin
The Olona is long and has a drainage basin of. The drainage basin of the Olona extends over part of the province of Varese, the Metropolitan City of Milan and, to a lesser extent, province of Como, also affecting part of Switzerland. A small part of the Gaggiolo basin, its tributary, in fact belongs to the Canton Ticino.With its, the Olona drainage basin occupies 5% of the Lombardy area and hosts approximately 1,000,000 inhabitants. The catchment area of Olona instead measures 370 km².
Any hydrographic engineers describe the Olona and the Lambro Meridionale as a single stream that flows into the Lambro at Sant'Angelo Lodigiano and which has an overall length of.
Original bed and the two Olona
One of the most important studies on hydrography of Milan was carried out by engineer Felice Poggi. In 1911 Poggi affirmed that the two Olona, the one that flows into the Lambro Meridionale and the stream that flows into the Po at San Zenone al Po, constituted until the first years of the Common Era a single river that had a total length of. This hypothesis has also been confirmed by subsequent studies.The place where the river was diverted to Milan by the ancient Romans is Lucernate, a fraction of Rho. From here, to find the ancient bed of the Olona following the minimum undulations and the very small altimetric variations of the terrain, we arrive at Cascina Olona, in Baggio and in Corsico, with a possible variant that from Settimo Milanese would lead to Muggiano and Trezzano sul Naviglio.
In the neighboring towns of Cesano Boscone, Corsico and Trezzano sul Naviglio, all three of which rise on the Naviglio Grande, it is possible to identify two waterways that could flow - towards the south - into the ancient natural riverbed of the Olona to Binasco: at Trezzano sul Naviglio and Cesano Boscone would be the Belgioioso canal, while in Corsico would be the Vecchia roggia.
From Binasco, with the derivation of the Canale Ticinello, we arrive then a little further south, in the territory of Lacchiarella, where the Colombana and Carona irrigation channels bring water to the irrigation network giving rise to the Roggione. The Roggione, when at the Settimo di Bornasco receives the Olonetta canal, changes its name to lower or Olona. The Olonetta, together with the Misana canal, comes from a fountain in Misano Olona, a few kilometers upstream. Taking the old course and its name, the southern Olona flows into the Po at San Zenone al Po.
The two Olona do not have an autonomous hydrography: at Rozzano, from the Lambro Meridionale, a branch take off towards the south-west, gaining vigor thanks to the water supply provided by springs and artificial canals. This stream then flows into the southern Olona. The reconnection of the two Olona is being planned with the construction of an artificial riverbed that would resume the ancient course of the river.
Detour to Milan
Until ancient Roman times, at La Maddalena, today's quartier of Milan, the Olona was diverted towards the city with the aim of bringing water to it: in ancient Roman times it flowed into the moat of the republican defensive walls of the city of Mediolanum then, from the 12th century, in the defensive moat around the medieval walls and later in the Darsena of Porta Ticinese.In particular Olona river, during the Middle Ages, flowed into the moat of medieval defensive walls of the city in correspondence of the modern Piazza della Resistenza Partigiana, while in ancient Roman times it continued the city route reaching the modern Piazza Vetra, where it poured its water into the moat of Roman walls thanks to the homonymous canal, the Canale Vetra. The Cerchia dei Navigli then originated from the medieval moat of walls, while the two branches of the Roman moat became the Grande Sevese and the Piccolo Sevese, two canals still existing today in the Milan underground.
The Milanese stretch of the Olona corresponds to the ancient natural beds of two streams, Bozzente and Pudiga. Before the deviation of the Olona towards Milan the Pudiga stream, after having lapped on the western side of the city, continued south following its natural bed, corresponding to what is now called Lambro Meridionale, which ended its course, like the ancient Pudiga, in the Lambro near Sant'Angelo Lodigiano. Originally, at the height of the inhabited center of Milan, the Pudiga made a wide bend towards the east, which led it to touch the city at the height of the modern Piazza Vetra, near the natural and ancient bed of the Nirone stream, and then bend towards the south following the bed of the modern Lambro Meridionale.
Originally, on its hydrographic left, the Pudiga, in place of the Olona, received the Bozzente stream. The Bozzente originally had in fact an autonomous natural bed that led him to collect the water of the Lura stream and the Merlata stream and then flow into the Pudiga. As already mentioned, it was the ancient Romans who diverted the Olona to Lucernate, a quartier of Rho, in the bed of Bozzente and then to the Pudiga riverbed. The Milanese stretch of the Olona therefore corresponds to the ancient natural beds of Bozzente and Pudiga. The new artificial riverbed of the Olona was then excavated from scratch only for a short distance: having reached the modern Lucernate at the Bozzente stream, the designers widened their bed to accommodate a greater water flow.
As the final destination of the new Olona route, the moat of Milan's Roman walls was chosen, where it poured its water into the Canale Vetra at the height of the modern and homonymous piazza: to achieve this goal, the ancient Romans extended and enlarged the Vetra canal towards the aforementioned Pudiga natural meander so as to also collect the Olona water. With the deviation of the Olona towards the wall of the city, the water continuity of the ancient bed of the Pudiga disappeared, whose southern section was intercepted remaining devoid of clean water, which came from the north, thus becoming in a sewer collector.
Until 1704 the river had only one terminal arm, while on a map of 1722 it is reported that the Olona forked into two almost parallel branches that met before entering Darsena of Milan: the Olona Nuova, that is the northern one that later will be called roggia Molinara, and the Olona Vecchia, that it was the southern one. The Molinara canal was then buried at the end of the 19th century before the river was channeled. The so-called "Brera island", which was located between the present-day Via George Washington and Via Vincenzo Foppa, had a longer life. Originated from another fork in the river, it took its name from the homonymous farmhouse that once stood there. It is still marked in a paper from 1925. In this context, in 1894, Canottieri Olona was founded, a multi-sports club in Milan, winner of an Italian men's water polo championship, based in the Darsena of Porta Ticinese.
In 1919, as part of the complex hydrophobic revision of Milan, the channels of the current Olona route began to be built, which involved the deviation of part of the river's water towards the Lambro Meridionale passing through the outer ring road. However, the branch that led to the Darsena was maintained. The detour to the latter took place in Piazza Tripoli: here there was a lock that diverted the river by Via Roncaglia, starting what was called the "Darsena branch". In the two dry periods of the Navigli annual, the sluice was maneuvered in such a way as to completely close the Darsena branch causing the entire flow of the Olona water to flow into Lambro Meridionale. The Lambro Meridionale, which at the time was a veritable sewer collector, was also known as "Lambro Merdario".
The new channeled route, which was also envisaged by the Beruto Plan of 1884, the first regulatory plan of Milan, did not come into operation until the early 1930s. The first covering carried out in Milan on the course of the Olona occurred in 1935 on part of the Darsena branch, when instead of the cattle station of the railways that stood there was built the Solari Park.
The remaining part of the Darsena branch, and the canalized section along the ring road, were instead covered between 1950 and 1970. With the passing of the years, and with the increasing pollution of the river, the sluice of piazza Tripoli was not maneuvered only to divert the flow of water during the dry of the Navigli: at first it considerably reduced the flow of the Darsena branch and, at the end of the 1980s, it was resetted for "hydrogeological risk and danger of pollution" of the Darsena and the water that came out for irrigation or navigation purposes.