How to isolate water block movement

In the pic below you see that I have a water block on the left that ends at the mountain pass - then no water block on the right but the elevation goes below the water block position on the left. So...when my character travels down into the valley on the right, he starts slowing down as if in water.

Is there a way to isloate the water movement effect to when the player is actually underneath the water block area?

rebelplanetcreations.com/Media/water_level.jpg