The Thames Barrier was built in the late seventies and early eighties, and was first put into operation in 1983. It is designed to stop tidal floods from engulfing the city, should the worst happen and it is the second largest structure of its kind in the world.
To add to the viewing experience, on the North side of the river is a 22 acre park, which according to the the London Development Agency's gushing website is home to "lawns, trees and uniquely contoured Yew and Maygreen hedges...an excellent children's play area, 5 a side football/basketball court, great places to picnic and play, the Thames path, magnificent views of the barrier, and a fountain plaza where 32 jets spring from the ground to provide a cooling and entertaining delight for children to splash and play."
The park also has a "Pavillion of Remembrance", which honours victims of the Blitz. There is also a wildflower meadow and a cafe. Perfect for a sunny day.
The park is open daily until approximately dusk and is beside Pontoon Dock DLR station. For more information on the park and barrier take a look at the Wikipedia page, or the page on the London Development Agency Website.