Hi Bartari,
Welcome to the forum. Hope you enjoy it...
Rivers separate fields the same as roads. However, you have to pay attention to large bridges, that join cities and do not split the fields. The field strips pass under them uninterrupted. For example:
You can find some clarifications about river tiles configurations and how they affect fields here:
https://wikicarpedia.com/index.php/River#Clarifications_for_new_landscape_tilesRegarding ferry lakes, they also serve a field boundary, the same as rivers. They are another body of water bordering on fields. The roads leading to ferry lakes end at a jetty connected by the ferry wooden piece. Moving the ferries does not change the configuration of the fields.
If you are interested in this possibility, a C1 fan expansion called
Detour & Impasse explores this concept of changing roads and field configurations during the game by using ferry pieces. It is like removing the lakes from the tiles but keeping the ferries connecting / disconnecting roads. As a result, these moving ferries affect the configuration of the neighboring fields too. This was a collaborative effort of several forum members and posted by CarKing in the Download area here:
http://www.carcassonnecentral.com/community/index.php?action=downloads;sa=view;down=267Cheers!