There was Once a Major Canal Along the National Mall

An 1862 map shows that the Washington City Canal ran along the north side of the mall, on what is today Constitution Avenue, toward the site of the Capitol Reflecting Pool (the Mall itself was much smaller, ending at the site of the Washington Monument):

This is a view from the Capitol Building looking West, you can see the Smithsonian Castle at center-right and Maryland Avenue SW at the left:

It existed from 1815 to 1871, when it was filled in. Here's another, illustrated, depiction: