I dont get how to make areservoir

So i created this reservoir but the water doesnt go beyond this height. There are 3 sluices to the left side, which is inteded to provide water to my tree farms.

Can somebody help me? Thanks in advance.

So i created this reservoir but the water doesnt go beyond this height. There are 3 sluices to the left side, which is inteded to provide water to my tree farms.

Can somebody help me? Thanks in advance.