Water 'funnels' in Minecraft

I've found a dungeon and would like to set up a automatic mob-item collector and am having trouble coalescing the items easily. How do I create a funnel of water such that I can go from a river N blocks wide to a river 1 block wide, and the items in the river will flow into the one block?

The simplest way to accomplish this is to drop the items from your many-blocks-wide channel into a channel running perpendicular to it, which can be any size you want. You do this by making a "terrace" eight blocks wide (water spreads seven blocks from its source or "waterfall", for a total of eight blocks).