Water flows into an area a Sponge is draining from

Was trying to drain a part of the river and the sponge didn't work. Unexpectedly, all the water flowed back into the spot,

What I expected to happen:-
All the water in a volume of 5x5x5 blocks in all directions should've been drained.

What really happened:-
The water was drained, but the water flowed back in, hence, causing no difference, and the water to not be drained

Steps to Reproduce:-
1) Put a sponge in a large enough volume of water,
2) Watch in amazement, as the water flows back in,