Canal's were built to connect navigatable stretches of rivers with towns, cities and each other. This was in the days when the only other method of transport were horses.
Long stretches of rivers were unsuitable for navigation but they could be used to supply a canal with water.
The volume of water in a canal can be controlled, so that only a serious drought can affect it's operation
At Chesterfield the Rother supplies the canal, and at the same time further along it's length takes back excess water. Why go to all this trouble?
We have all seen river's in flood, and the devastation this causes. Floodgates prevent the canal system being washed away by floods and spillways allow excess surface water to escape. There is another reason "LOCKS"
If you are sailing on a river you either go upstream or downstream. The locks on a canal allow you to go up or down whilst travelling in the same direction. A lock is made up of at least two gates, the water between these gates can be raised or lowered. Sections between locks are level so that they are easy to navigate, horses pulled the barges so the flow of water in the canal had to be at a minimum.
Canals are, like railways, a remarkable feat of engineering when you consider that there were no mechanical excavators. Today lasers and satelites are used for levelling. It is hard to imagine the workforce today being prepared to work under the conditions of that time.The decline of the canal system due to the railways and improved roads may have left us with an expensive restoration programme, but at the same time the decline probably prevented more industrialisation on their banks. Today the leisure industry benefits from this because the canals pass through open countryside and woodland. Far more appealing than large factories e.t.c.