The Happy Pontist correctly stated that the bridge pontoons are anchored to the channel bottom. This area is affected by tidal currents so the anchorage must keep the bridge from lifting off the quays during high tide. He also correctly named this bridge. I was originally reluctant to give the proper name because I didn't know if I would be able to continue to do so for all the bridges around Canary Wharf.
Great Britain: West India Quay Footbridge (2) by Mark Yashinsky is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 3.0 United States License.