Port of London Authority:
"On the weekend of the 10th and 11th March 2012 the rigging of cables
across the river between the North and South Towers will be undertaken
by contractors; working on behalf of The Docklands Light Railway. This
work continues the GLA project to erect a Cable Car across Bugsbys
Reach in the vicinity of Blackwall Point."

What have the Docklands Light Railway got to do with this? Serco
operating the "Air Line" for Emirates/TFL?

The 2 page PDF Notice to Mariners issued by the PLA detailing closures
of the river to traffic (with timings) is he
http://tinyurl.com/79jwrwu
or
http://www.pla.co.uk/notice2mariners...of_Cables1.pdf