U.S. citizens living in Canada have an obligation to file US tax returns.
U.S. citizens are required to file a U.S. tax return regardless of where they live in the world. There are however some notable out-of-country exemptions available, and you will want to take advantage of these to minimize or even completely eliminate your U.S. tax bill.
You also have to report asset holding to both the IRS on your tax return, as well as to the U.S. department of treasury using a completely separate system.
An area of caution here is that not every item such as RRSP, TFSA, RESP, and the list goes on and on, are treated equally under both tax systems. The U.S-Canada Tax Treaty covers a lot of items to reduce double-taxation, but not every single item. So as an U.S. citizen living in Canada be careful what types of accounts you hold here in Canada, and talk to some who knows what to avoid.