Is Cash Flow One Word or Two?

This is an issue that I recently came across when writing up the finances for a report. I was in charge of discounted cash flows so I needed to figure out whether it was "cash flow" or "cashflow". There appeared to be no simple definitive answer so I decided to write one. My conclusion is:

Although both are used cash flow is much more commonly used and so should be used.  

Tellingly performing a Google search for "cashflow" gives results that all use "cash flow" with the exception of brand names. 

Results obtained from searching "cashflow". I'm logged into two Google accounts which is why I have to use incognito (before you start accusing me of anything)

Furthermore when I took a look at the annual reports of a few companies such as Drax Plc, Lancashire Holdings Ltd and Standard Chartered Plc they all use cash flow. Hence I would say that if you use cash flow you are in good company. 

I guess this is a matter of prescriptive (how it should be) vs descriptive grammar (how it is actually understood). If we take a descriptive approach its clear that cash flow is the one that is best used.  If you stick with "cash flow" in your reports you should be fine. If you are challenged on it just quote some of the references I've given you.