Is there a way to create a water item-slide from a creature-trap without chickens passing through it in Minecraft?

I have created a vertical trap that uses water to drag unsuspecting creatures from the surface into a vertical pit, they literally fall to death.

Well, except chickens, and sometimes spiders.

Right now I have to open a door and enter the trap periodically to recover loot from the creatures.

I would like to set up a water "slide" at the bottom that would make items dropped by dead mobs flow with the water out of the trap and to where I stand. However, to do this I have to open one cube of the bottom of the trap for the water to flow through, and apparently that's enough for chickens to get past. I tried this, and when I got back from dinner, I had about 15 chickens flying around me, outside the trap.

I have created an elaborate drowning tool to handle chickens and spiders that survive, but this of course only works if the mob is still in the trap, and not roaming around outside, and I have to manually activate and deactivate this, so it's not automatic. However, if the spiders and chickens could just stay in the trap, I have no problems with activating this from time to time.

However, I don't want to go chicken-hunting in the cramped area outside the trap.

Is there any way I can set up such a water slide, get the items to exit the trap, without getting the outside of my trap filled with chickens?

Basically, the bottom of my slide, seen from above, was like this:

RRRR  <-- R is rock
R WR  <-- W is a water source, this will flow down and to the left
R  R
R  R  <-- vertical shaft is over
R  R  <-- this 2x2 area (the spaces between the rock)
R  R
R RR
RSRR  <-- S is where I stand

In this case, everything is covered up, except that the cube just above the S has to be open for the water to pass, and thus the chickens can escape.

Is there any way for me to get the loot, without opening up for chickens?

put a slab on the square you are standing on and leave a 1 square gap in the direction the water is flowing from (use glass if you want to be able to see what's coming). The slab will prevent anything from being pushed through but you will automatically pick up the items as they approach.

Adding a picture to clarify:

Side view:

 G   W
UG   W
UG   W
SWWWWW

W = water
  = blank space
G = glass
U = where you stand
S = Slab

The chickens won't be able to move into where you are due to the half slab keeping them in the water tile beneath the glass.

I tried various of the existing answers here, and they didn't really solve anything.

• The slab answer didn't quite work, but since it was the one that sparked my solution, I accept that one. In short, the items ended up butting against the slab, and was apparently just out of reach for me.
• I did not get to check the auto-drowning system that Michael posted, it requires some extensive reworking of my current trap design.
• The cactus idea doesn't work, as the cactus can't be placed like suggested (at least not in 1.7.3 beta)

However, what did work was to change the original idea just slightly.

RRRR  <-- R is rock
R WR  <-- W is a water source, this will flow down and to the left
R  R
R  R  <-- vertical shaft is over
R  R  <-- this 2x2 area (the spaces between the rock)
R  R
R RR
SRRR  <-- S is where I stand

The difference here, from what I had in my question, is that I am standing one square to the left of my original idea. However, since the water flows towards me, all items will end up in the corner butting against me, and are thus just in reach to pick up, even though there is a wall between me and them. This solved the chicken problem, as they now have nowhere to escape.

One option is to remove the chickens from the rest of the mobs during their fall down the vertical shaft. Since chickens gracefully float down instead of plummeting to their doom like other mobs, a stream of water will catch them, whereas other mobs will fall through it. If you build a water stream like in this screenshot (the water is supported by signs), you can filter out all the chickens, and put them in a separate room (perhaps a egg farm).

In this example, the mobs would fall down the chute on the right, and the chickens would get caught on the water stream. The other mobs would pass through and fall into the right hole, but the chickens would float into the left hole instead.