How come water stays in a canal and doesn’t seep into the ground?

Much depends on the subsoil in which the canal is constructed and the level of the groundwater level.

If the groundwater level is usually higher than the water level in the canal, there is no problem and the groundwater feeds the canal.

If there is little difference between the two levels, then there is no problem, because then there is little exchange.

If the groundwater level is lower than the water level in the canal and the subsoil is rather impermeable (clay and clay – loamy soil), little water will drain away. However, if the subsoil is permeable (sand or gravel), then in principle an impermeable layer is applied to the channel bottom.

If losses are too great, sufficient water supply is provided to compensate for the losses.

Note that a river can also flow through an area with a relatively permeable subsoil and release quite a lot of water into the groundwater there.