The Adda flows out of the lake at its south-eastern extremity at Lecco, and has thence a course through the plain of above 70 m.
Two other small lakes in the same neighbourhood, as well as those of Erba and Pusiano, between Como and Lecco, are of a similar character.
It is connected with Milan by two lines of railway, one via Monza (the main line, which goes on to Chiasso - Swiss frontier - and the St Gotthard), the other via Saronno and also with Lecco and Varese.
Railways radiate from it to Lecco, Ponte della Selva, Usmate (for Monza or Seregno), Treviglio (on the main line from Milan to Verona and Venice) and (via Rovato) to Brescia, and steam tramways to Treviglio, Sarnico and Soncino.
On issuing from its south-eastern or Lecco arm, it crosses the plain of Lombardy, and finally, after a course of about 150 m., joins the Po, 8 m.