This is a problem that I have dealt with a couple of times (at least) in the last few months. Sooner or later your contributions (no later than the year you turn 71) must cease. At that point your RRSP (Registered Retirement Savings Plan) savings must be converted to income. A RRIF (Registered Retirement Income Fund) is a popular way to convert that RRSP to income.
Just remember that by creating a RRIF you are opening a new account and will be subject to the FATCA inquisition. U.S. citizenship creates planning problems to be solved!