Canals

The Canals are a network of rivers and drainage tunnels which served as both drainage and possibly transportation.

Features
The canals near or in City 17 appears to be heavily used as drainage, and has a very large network of sewers and waste tunnels. Because of the Combine invasion and the drop of earth's resources, the canals near the city had almost all drained out, and what remains is the riverbed and a large amount of waste.

Outside City 17, the canals appear to be more rivers and waterways than tunnels and sewers, and was probably used more for transportation than drainage and waste disposal. The water levels, however, are low, and following the Combine invasion, a large portion of the canals in both the city and outside had become toxic waste or other hazardous materials.

Civil Protection units frequently patrolled the canals for resistance activity, and were tasked with maintaining the many useful buildings and outposts located along the rivers.

Resistance presence
The resistance had established an organised system of stations for refugees hoping to escape City 17. Through the use of radios and hidden bases containing supplies and living quarters, the resistance had formed a rather strong foothold in the canal areas.

Resistance used both the canals inside City 17 and outside, using the many tunnels and drainage areas to secretly establish living areas and bases.

Half Life 2
With the arrival of Gordon Freeman, Civil Protection was fully aware of the presence of resistance refugee stations, and were conducting raids on the stations and killing resistance personelle. Eventually all the stations were either found and destroyed, or were closed down.