There are also lockers located outside of DCA to the left of the entrance gates. Not many people use those so if you can't find them inside the park or outside entrance of DL you will likley find one over there.
There are large lockers in the picnic area-- but they cost more than the other locations.
You can use cash or credit card to pay for lockers. It will print out a receipt and give you access code to your locker (keyless) , allowing multiple entry/exit throughout the day.
Pick a locker at beginning of day and use it until you head out of park at night. Sometimes we are nice and put the receipt/code peeking out of a locker so someone else can use it for free
