Why is the Potomac so Dirty?

The first two-thirds of the Potomac were wild and clean, but the closer we got to Washington DC,  the more debris we saw.

As we passed Mount Vernon and Fort Washington we saw rafts of branches and logs swirling in the current. Even though we tried to avoid them, our eyes would widen with alarm each time we thumped upon a submerged log.

Around the National War College the logs disappeared and a new kind of debris popped up; plain old garbage, much of it plastic.

When we arrived at the dinghy dock we were appalled at the trash. Washington DC is a clean city, so how does all this garbage get into the water?

Back at the boat we enjoyed the view, but still want to know: Why is the Potomac so dirty?