While working on a project in the overworld that involved souls sand and flowing water, whenever the water source is removed, water continues to flow on the first soul sand blocks after the water fell from a higher block, leading to blocks having to be placed for the water to be removed.  In the picture shown, the pistons are holding back the true water sources.  The water that is still flowing on the lower level of soul sand isn't supposed to be there.  The mid-level of soul sand is only missing as I placed blocks to remove the water.