The Oglio (Italian: [ˈɔʎʎo]; Latin Ollius, or Olius; Lombard Òi; Cremonese Ùi) is a left-side tributary of the river Po in Lombardy, Italy. It is 280 kilometres (170 mi) long.
In the hierarchy of the Po's tributaries, with its 280 kilometres (170 mi) of length, it occupies the 2nd place per length (after the river Adda), while it is the 4th per basin surface (after Tanaro, Adda and Ticino), and the 3rd per average discharge at the mouth (after Ticino and Adda).
The Oglio is formed from the confluence of two mountain streams, the Narcanello branch from the Presena Glacier, in the Adamello group and the Frigidolfo branch, in the Corno dei Tre Signori, part of the Stelvio National Park. The streams merge near Pezzo di Ponte di Legno, both the streams have an average discharge of 1.5 m3/s (53 cu ft/s).
The Frigidolfo branch, before merging with Narcanello branch, receives the Arcanello branch, which have an average discharge of 0.5 m3/s (18 cu ft/s), which receives a minor branch originating from Lake Ercavallo.
The Ogliolo stream, with an average discharge of 2.5 m3/s (88 cu ft/s) and a length of 20 kilometres (12 mi) (similar to the length of the main branch between the Frigidolfo-Narcanello merge at Ponte Di Legno and the Oglio-Ogliolo merge at Edolo) is sometimes considered as a de facto fourth branch of the Oglio river.
It flows in a southwest direction, through Valcamonica and enters into the Lake Iseo at Costa Volpino. It leaves Lake Iseo at Sarnico and, after traveling a zone of moraine deposits, it joins the Po river at Torredoglio, not far from Cesole and Scorzarolo, in the province of Mantua. Its drainage basin, which corresponds to the region of Valle Camonica, covers 6,649 square kilometres (2,567 sq mi). It is part of the larger Po-Adige basin.
The course of the river main branch
Camonica Valley section
The river in the Camonica Valley flows almost exclusively in the province of Brescia, entering in the province of Bergamo only at 45°51′38.2″N 10°08′56″E, on the border between Artogne, Brescia and Rogno, Bergamo.
The following is a list (incomplete) of the tributaries in the Camonica Valley:
Frigidolfo and Narcanello branches, also known as Oglio Frigidolfo and Oglio Narcanello, respectively, in Ponte di Legno at 46°15′32.7″N 10°30′28.7″E merge forming the main branch
Valpaghera, at Vezza d'Oglio 46°14′10″N 10°23′59″E
Valgrande, at Vezza d'Oglio 46°14′07″N 10°23′48″E
Ogliolo branch, at Edolo 46°10′18.2″N 10°20′03″E
Rabbia, at Sonico 46°09′01″N 10°20′44″E
Remulo, at the border between Sonico and Malonno 46°07′43″N 10°19′56″E
Allione, at the border between Malonno and Berzo Demo 46°05′24″N 10°18′58″E
Poia river, at Cedegolo 46°04′37″N 10°20′54″E
Re, at Sellero 46°03′31″N 10°21′05″E
Re, at Capo di Ponte 46°02′10″N 10°20′43″E
Clegna, at Capo di Ponte 46°01′35.5″N 10°20′38.6″E
Figna, at municipal border between Nadro, Ceto and Ono San Pietro 46°00′45″N 10°20′37″E
Blé stream, at municipal border between Cerveno and Ceto 46°00′32″N 10°20′34″E
Palobbia creek, at municipal border between Ceto and Braone, near the quadruple municipal border with Losine and Cerveno 45°59′35.5″N 10°20′04.6″E
Poia creek, at Losine 45°58′58.3″N 10°19′15.4″E
Re, at Losine, near the municipal border between with Niardo 45°58′35″N 10°19′02″E
Lanico stream, at the municipal border between Malegno and Cividate Camuno 45°57′05.5″N 10°16′42″E
Trobiolo, at Cividate Camuno, near the municipal border with Cogno, Piancogno 45°56′03″N 10°14′49.5″E
Grigna stream, Esine near the municipal border with Piancogno 45°55′14.3″N 10°13′54.7″E
Resio, at Esine 45°54′25″N 10°13′22″E
Davine river, at Darfo Boario Terme 45°53′16″N 10°11′04″EBudrio stream, at Erbanno, Darfo Boario Terme 45°53′56.1″N 10°12′00.5″EDezzo river, at Darfo Boario Terme 45°53′16″N 10°11′04″EGleno, at the triple municipal border between Vilminore di Scalve, Colere and AzzoneRe stream, at Darfo Boario Terme 45°52′29.9″N 10°09′52″E
Orso creek, at Rogno 45°50′57″N 10°08′06″E
Supine creek, at Costa Volpino 45°49′40″N 10°06′16″E
Lake Iseo section
At Costa Volpino, the Oglio river enters the Iseo lake with a mixed delta-estuary mouth.
Po Valley section
The Oglio main branch leaves the lake at the provincial border between Sarnico, Bergamo and Paratico, Brescia 45°39′56.5″N 09°57′24″E and begins to follow (approximately) this provincial border.
At the triple provincial border between Torre Pallavicina (Bergamo), Roccafranca (Brescia) and Soncino (Cremona) 45°25′40.5″N 09°53′07.5″E the Oglio begins to follow (approximately) the provincial border between the Brescia-Cremona provincial border.
The major tributaries in the Po valley are the following:
Cherio river, at the provincial border between Palosco, Bergamo and Pontoglio, Brescia 45°34′44.5″N 09°50′30.53″E
Mella river, at the provincial border and triple municipal border between Seniga (Brescia), Ostiano (Brescia), and Gabbioneta-Binanuova (Cremona) 45°13′30″N 10°13′10″E
Chiese river, at the triple provincial border between Canneto sull'Oglio (Brescia), Acquanegra sul Chiese (Mantua) and Calvatone (Cremona) 45°08′28.01″N 10°25′17.45″EAt this triple provincial border the Oglio river begins following the Mantua-Cremona provincial border, which leaves at the triple municipal border between Acquanegra sul Chiese (Mantua), Calvatone (Cremona) and Bozzolo (Mantua) 45°07′57″N 10°29′32″E where the river enters entirely in the Mantua province.
At the municipal border between Marcaria and Viadana 45°02′40″N 10°39′37″E, the Oglio river enters the Po river.