Building one directional valve

Solution 1:

To summarize the comments, it can be done but needs at least two z levels.

Z+1    Z

+++   ++7
+ +   +7+
+P+   +++ 
+p+   +++
+ +   +7+
+++   +7+

+: Wall
P: Pump output
p: Pump input
7: Water

Water will taken from south, and exits to the north. The diagonal gap is required to remove the pressure from the water stream, otherwise it would flood onto Z+1 level whenever possible. Because this slows the stream very much, in some cases it might be better to avoid the gap (water will only flood Z+1, not Z+2) or place it further downstream.

Can be combined with a water wheel for automatic operation.

 Z+1    Z
 +++   ++7
 +w+   +7+
o-w+   +7+
|+w+   +7+
o-P+   +++ 
 +p+   +++
 + +   +7+
 +++   +7+

+: Wall
P: Pump output
p: Pump input
7: Water
w: Water wheel
-, |: Axles
o: Gear

Might cause slight spillage of water around the upper axle though.