The Lincoln Memorial Reflecting Pool is one of the most famous attractions in DC.
While it is technically part of the Lincoln Memorial, it deserves its own post.

What is a Reflecting Pool?
Reflecting pools are common at memorials - the still water represents calmness and serenity. It is a place of contemplation and reflection.
It is also in a direct line with the Washington Monument so it perfectly reflects most of the monument.
Lore says that the only way you could see the entire monument reflected is if you were level with Lincoln's eyes but don't climb up and check for yourself!
There are no fish in the reflecting pool, though there are fish in the nearby also manmade Constitution Gardens pond.

You'll often see ducks in the reflecting pool.
There is even a volunteer group to help baby ducks who get stuck in the waterfalls at the nearby WWII Memorial and put them back in the reflecting pool.
Other reflecting pools can be found at the Korean Veterans Memorial, FDR Memorial, and the US Capitol among others.

How long is the Lincoln Memorial Reflecting Pool?
It is 2,029 feet long which is about a third of a mile.
The reflecting pool is not as deep as it looks. It is only 18-30" deep depending on how close to the center you are.
Which means Jenny really did run through during the filming of Forrest Gump!
Unfortunately, you are not permitted to re-enact that scene.

Can you swim in the Reflecting Pool?
No. It is both not allowed and due to the shallow water, not even possible.
You are also discouraged from wading or walking in it. One some of the hottest afternoons you may see people dipping their feet in but it doesn't take long for them to notice how dirty the water is and pull them out.
In the past you used to see much more activity on the reflecting pool. Summer wading and sailboats and winter ice skating. None of that is allowed now but sometimes you'll see people try out the strength of the ice on cold days.

Why is the Reflecting Pool empty?
The entire pool was built in 1922 after the dedication of the Lincoln Memorial.
In 2010 the entire pool was dug up and rebuilt. It has been continuously drained and cleaned since then.
In order to maintain the filters and the pool itself, it is regularly cleaned. The water actual has a small current that was meant to keep algae from forming, but it doesn't seem to be working.