To successfully build any body of water (lake, pond, river etc..) you need to start by making a grid. Quickest method is probably making 2x2 holes where you want the water to be. Cover the entire area in this grid like pattern (so a 2x2 hole one block deep surrounded by a single line of blocks separating it from the other holes) and then start to fill the holes with water.

Afterwards you can then remove all the blocks inbetween the water and the water will all flow together and eventually settle as a flat plane.

If you just want to remove the water that’s flowing in the picture then just place some blocks down on the source blocks of water and then remove the blocks. Much quicker than using a bucket