Hi Erdem. You can use Digital Elevation Model (DEM) of your consider area or basin. By the use of ArcGIS software>> the section of Hydrology, you can draw the maps of flow direction and flow path for surface waters based on the direction of slope achieved from DEM.
Hi Erdem. I have done this using two methods depending on the system and data availability. The answers about using a DEM are correct and in the UK we now have a recent DEM of high accuracy and at 5m postings. However more historically digitising linear water features to encode flow direction has been carried out by setting up a rule for this feature type in the system. The rule stated that digitisation was always in the upstream direction and end node codes used to manage the connectivity.
The problem of define flow direction is very easy when you have a data on local topography
In many cases when there is a flat topography it's can be tricky
You can use the elevation profile from google earth to find the general elevation profile along your canal , the other option is to use the data from DEM. There is a good DEM from the Japan Aerospace Exploration Agency (JAXA)
I would start by looking where the water sources are that supply the canal and their relationship with locks this way you should be able to get the hydraulic gradient without too muchfield testing