Either of you can terminate the tenancy, consistent with the terms of your rental agreement. If you're speaking of a lease with an annual term, not a month-to-month lease, normally a lease will hold co-tenants jointly responsible for rent until they both vacate. If your roommate gives notice, you may have rights under the lease to continue the tenancy without her. You don't have to give notice even though your roommate would prefer that you move out.
If your lease provides for automatic renewal, and you do not give notice such that the lease automatically renews for a one-year term or if it becomes a month-to-month lease, you will continue to be co-tenants. If you are required to actively renew the lease -- sign a renewal agreement -- in order to continue the tenancy, then your landlord will have to decide whether he wants to allow the lease to go month-to-month or ask you both to move out (backed up by the possibility of eviction).
All of that said, this really does boil down to a matter that your landlord can leave you to resolve with your roommate. The only real question from the landlord's perspective is, if the lease doesn't automatically renew for a one-year term, whether he would ask you both to leave or if he would let the lease go month-to-month.

