It does define impeachment as a method to remove federal officers, it does not explicitly state that no other method exists.
"The President, Vice President and all civil Officers of the United States, shall be removed from Office on Impeachment for, and Conviction of, Treason, Bribery, or other high Crimes and Misdemeanors. "
It is an open question if the phrase "all civil Officers of the United States" include representatives and senators. Only one congress critter has been subkect to an impeachment,(The 1799 impeachment of Tennessee Senator William Blount
http://en.wikipedia.org/wiki/Impeachment_in_the_United_States) and that case never went anywhere as Blount was 'expelled' (which would imply there are other ways to get rid of a senator) and the case was dismissed. So far this issue has never been tested.
In fact the Senate thinks it can expell its own members outside of the impeachment process entirely.
"United States Senate Election, Expulsion and Censure Cases
Senate Historical Office (1995)
Article I, section 5 of the U.S. Constitution provides that each chamber of Congress “shall be the Judge of the Elections, Returns and Qualifications of its own Members” and may “punish its Members for disorderly Behavior.” Over its more than 200-year history, the Senate has developed procedures for judging the qualifications of its members in contested elections and for taking disciplinary action against Senators through such measures as formal censure or actual expulsion from the Senate. U.S. Senate Election, Expulsion and Censure Cases discusses in detail the 141 major cases of contested elections and disciplinary cases in the U.S. Senate from 1793 to 1990. "
http://www.senate.gov/reference/reference_item/election_book.htm