Maps contain land and water areas. Almost all construction in a city happens on land, although bridges and crossings can span bodies of water, and some structures are built on the shoreline or in the water.
Water fills channels and basins and flows downstream. The direction and speed of flow is indicated by white arrows in the water and sewage info view. The game will also simulate more dramatic water physics, such as waterfalls. as well as unstable phenomena, such as large waves. The latter are most likely to occur when there is a dam-related catastrophe, a tsunami, or through the use of the terraforming tools in the water or along the shoreline. If a building becomes flooded and remains that way, it will soon become abandoned. The Pumping Service building from the Natural Disasters DLC dispatches vacuum trucks to clean up flood waters.
Mayors need to provide their city with water and sewage services. Most cities will extract water using shoreline water pumping stations to provide water for the citizens. Water towers extract groundwater, but are a more expensive option. It is critical that the towers not be placed on polluted ground, or the citizens will quickly fall ill. Most cities will need to discharge sewage into the map's water bodies. This should be done as far downstream on a river as possible. The Sunset Harbor expansion added some Inland Water Treatment options that do not require bodies of water to dump sewage into. All water in Cities:Skylines is fresh water, however mayors should be careful not to pump sewage into the water grid. When water is polluted, it becomes brown. Polluted water may be cleaned up using Floating Garbage Collectors from the Green Cities DLC. Green Cities also includes sewage treatment options to reduce the amount of pollution at sewage outfalls.
Care must be taken with water pumping and sewer water draining because they may affect the water level. This depends on the natural water source and drain capability. Smaller rivers or lakes will be more easily affected by the water usage. Water towers, while more expensive and sensitive to ground pollution, will never affect the water level. Reducing water consumption and thus the sewer water drained will lessen the effect.