The entire surface of Mars was once criss-crossed with canals of varying sizes and lengths but only about 40% still have water and are navigable. The reminder have either dried up, are in hostile territory or, where they have been cut through mountain passes, have been blocked by falling debris from the cuttings. With the Martian population drastically cut when the Great Storm hit there hasn't been the manpower to keep up a continuous repair.
The canals that are still used are kept in a usable state as much as can be and there have been City Martian clan discussions about trying to extend the network and find some of the lost cities. Opposed by some but favoured by others this might become a possibility with the human presence and their ever increasing manpower levels.
The canals tend to be of a standard size all over the planet with enough room to allow two Martian barges to pass with ease. They are stone lined and various smaller branches and docks can be found over the entire network. Martian barges are generally a two story affair with the lower floor for heavier goods and the lighter ones on the top deck. Passengers can also be taken for a small fee or a turn at the punting poles. Humans have even started to take 'pleasure rides' on the barges, much to the amusement and sometimes annoyance of the owners. The barges are propelled with long poles thrust into the middling depths of the canals and along the stone side walls. Fishing sometimes takes place but the levels of fish seem to be dropping with each passing year; again, the reason is unknown.
Toll bridges can be found where land trade routes cross the canals and the toll owners are generally a single family and can be quite heavily defended; not against the trade caravans, who's relation with the toll families is generally a good one, but against the Sky Martians who will quite often prey on the lone bridge stations. The bridge stations themselves consist generally of two large, tall buildings, one on either side of e canal with an attached small temple (a common feature in a lot of Martian buildings), a signal-gem station on top, a ballista or two and the bridge inbetween. There is a ramp leading upto a heavy wooden door with an internal drop-bridge in each tower which can be used to bar passage. Another heavy door on the other side of the tower leads on to the bridge itself which is a high arched stone affair that can generally only allow one barge under at a time. Quite often, a ballista will also be mounted on the bridge.
The water in the canals is a murky green/brown colour and is full of minerals which the City Martians use to drink, although it does need to be strained of some of the larger lumps... Strangely, not many plants can be found alongside the canals as they appear to prefer the purer water from aquifers in the mountains although once the water has been boiled it can be used to water crops.
There are several types of fish that inhabit the canals and these can supplement a barges stores if the journey is long but as mentioned, these seem to be getting less and less in number. The largest is about three-four foot long and is fairly docile but some of the smaller types can leave a nasty bite if unwary fingers are dipped in the waters for too long. There have been rumours of large reptiles or somesuch in the more remote canals but whether this is a tale told by the punters to while away passage or actual fact, who can say.
