The highest point of the drainage basin is the summit of la Spedla (a subpeak of Piz Bernina), at 4,020 metres (13,190 ft).

Towns along the river Adda include Bormio, Tirano, Sondrio, Bellagio and Lecco (both on Lake Como), Brivio and Lodi.

The Adda's true source is in Alpisella valley near the head of the Fraele glen, but its volume is increased by the union with several smaller streams, near the town of Bormio, at the Rhaetian Alps.

Thence it flows first southwest, then due west, through the fertile Valtellina, passing Tirano, where the Poschiavino falls in on the right bank, and Sondrio, where the Mallero joins, also on the right.

The lower course of the Adda was formerly the border between the Republic of Venice and the Duchy of Milan, after the Treaty of Lodi, 1454; and on its banks several important battles have been fought, notably that of Lodi, where Napoleon defeated the Austrians in 1796;[2] several battles have also taken place at the bridgehead of Cassano d'Adda and surrounding countryside: Prince Eugene of Savoy fought the French here in 1705, and Count Alexander Suvorov forced the river in 1799.