Your goal is to build Lakes and place Ferries in them to score points. Lakes also have another purpose: you can send a route into the lake side of a tile to create a Harbor (a bit like Canal End tokens work in the Canals Expansion), and all routes that are connected to the same Lake via Harbors are also connected to each other... basically, Lakes act as big Stations!

Another special feature of the Lake Expansion also lies in the way Ferries are placed. At the end of each round, when you look at the active Placement tokens and place pawns accordingly, you must also look at each one of your Lakes: if you have more Harbors than Ferries in a certain Lake, you may place 1 Ferry pawn for free on any empty Ferry pin point until you have an equal number of Ferries and Harbors in that Lake.

You score 1 point +1 point for each Ferry in the same Lake, up to a maximum of 3 Prestige points (they have a lower point limit than other pawns, which can be worth up to 5 points each, but you also place them for free on top of those coming from the 2 Placement tokens, instead of in place of 1 of them).

Like all expansions, the Lake Expansion comes with a few Objectives as well: for example, the Fishermen's Island must be reachable by at least 3 Traveler pawns, while the Hydroelectric Plant must be part of a closed Lake with no open sides.

—description from the publisher