Rivers flow through the landscape making up natural obstacles. Bridges can be constructed over rivers in order for your citizens to get to the other side. Rivers provide a source of water, which can be used to put out fires. Rivers are, as well as lakes, suitable for fishing, providing food for your citizens once a Fishing Dock have been constructed and fishermen have been assigned to work there. Trading Posts can also be constructed along rivers in order to let the player trade.
There are two types of rivers: Large and small. Large, or wide, rivers is the type of rivers that merchants can access, meaning you can only build Trading Posts upon a large river. Small rivers don't support trading but works like large rivers in every other way. Small rivers still support fishing and you still need to build bridges in order to cross them.