It would be just as easy to cut the branches on the water side and trim the other to allow shade and protection. I doubt they will grow sufficiently in one summer to give any sort of protection against this glorious four month non-stop summer heatwave.To be fair the Willow trees grow ridiculously fast and are cut/ pollarded every couple of years as they reach the water line, those hanging branches will have grown back in no time at all.
S'good job I've got me hanky at the ready.