An abandon aqueduct

When we exited lock 25 earlier today, we emerged from the lock doors
into a field of essentially pond scum. Fortunately, it was mostly just
scum without too much weeds. After that however, the water was
occasionally mined with patches o weeds floating on the surface, but
far enough apart to navigate through without picking any up on the
prop.

Now that we're passing the old aqueduct, the water seems to be
clearing out again. Hard to believe water passed along the top of
that aqueduct. Didn't the Romans invent the Aquaduct?