It is very difficult to explain this kind of thing without creating drawings. Look, any ponds are very small. So, it has only small capacity to hold water.
Say, the pond is only (1km x 2km x 10m deep). This pond can retain only V1 = 20,000,000 m^3 of water. How many do you want to dig? Please do your calculation. But, note that the length is no more than 95 km.
Now, if there is a barrage at the mouth, the entire system will retain water measuring,, V2 = (95,000 m Length x 2,000 m wide x 18 m deep). It is V2 = 3,420 million m^3 of water.
V2 is 171 times larger than V1. Note also the surface of pond water will remain very low needing pumping up the water. A barrage will need almost no pumping because the water surface will reach (Dike Height - 1.5 m). Farmlands are about 10m below this level.
I am talking sincerely but most guys are not familiar with the calculations that I have shown here. You have to develop something in your brain/ eyes so that you can understand 3-dimensional views. 2-dimensional views are not good enough to understand this thing.
