What is the simplest, most reliable and cheapest way to control the filling of the tank?

Question

C++ / C#

Introductory: there is a water supply station located in Kamchatka in a small village. It takes water from the river and pumps it into the reservoir, from which it is fed by gravity to consumers. The reservoir is located 3 km from the village and from the station, electricity is not supplied there. At present, the pumps simply work "for pouring", it is necessary to make a system that controls the filling to the level and its maintenance. Cellular communication is not reliable, it is supposedly too expensive to lay a cable.
UPD: the solution should be energy efficient, pumps of 100 kW each, I would not want to spin them for nothing on the bypass.
UPD2: the diameters there are not domestic, if you install a mechanical valve, then you need approximately DN200

Answer 1

Have you seen the toilet bowl?

Answer 2

What period of arrival to the object will suit you? Or do you want to be powered by solar panels?

Answer 3

Is there a line of sight between the pump room and the tank?
Required antenna height for line of sight?
The intensity of the sun and wind in the area of ​​the reservoir?
The easiest and most budgetary way: put electronics on the tank, power it from the battery + SES / WES and send the status to the pumping station via the radio channel.

Answer 4

Look, make a hole at the very top of the tank, and hammer a hose leading to the river into the hole. When the water reaches this hole, it will automatically pour into the river! And cut off the pumps when water appears in the hose! That's all!

Answer 5

Water level sensors with a battery-powered controller, recharging from solar panels, and transmit data via radio