Log into your Googlemail account.
Click Forwarding and POP.
Under POP DOWNLOAD
2. When messages are accessed with POP
use pull down menu to select delete....copy.
But before you do this, and unless you have a dislike of your emails being stored on line or don't trust Google, I would chose the option to archive ... copy when downloading by POP, what will happen then is that you get a copy of your email into your Outlook then Googlemail stores a copy in your webmail (or 'archived'). This is a great idea in my view and something not many email providers do, essentially you have a retrieve and backup email service in one, so if your computer crashes and data is not recoverable, your emails are safe and can be re-downloaded again from Googlemail.
Another option to consider would be to use Googlemail's IMAP to handle email. IMAP basically means you manipulate emails on the server, rather than download them from the server. Personally, I always use IMAP.