This weekend’s Head of the Charles Regatta brought with it more than just spandex-clad rowers and crowded bridges; for Harvard Square establishments, it also brought a boom in sales and a change of pace from business as usual. Attracting more than 11,000 rowers of all ages and more than 400,000 spectators each year, the Head of the Charles Regatta is the world’s largest two-day rowing event. It regularly drives traffic to Harvard Square’s businesses, especially those in the hotel and food industries.