Sankey diagrams can be useful to show flows.
They are tricky to make in PowerPoint. The width of the arrow needs to correspond with the value of the stream. The curves of 90 degree arrows in PowerPoint are hard to control. If there is no escaping (maybe you can create a waterfall diagram instead), I create Sankey diagrams using boxes and triangles, see the example below.
UPDATE: I have added a PowerPoint template for a Sankey diagram in the SlideMagic template store: