Rivers, streams, and lakes.
Buses in opposite directions may follow completely different routes. Don’t assume that if a bus-line goes in one direction on a two way boulevard, it will also go in the opposite direction. What’s more, trolley-buses and buses may have the same number (e.g., 11) but they follow different routes.
On the excellent Sofia map - you can get from the tourist information in St. Kliment Ohridski Sofia University (Климент Охридски) metro station (42.688402, 23.338826) the buses’ trolley-buses, and trams are marked in different colours.