Coro Allegro joins the Boston Landmarks Orchestra, the One City Chorale, members of the New England Spiritual Ensemble, and the Boston String Academy for "Deep River," a free concert featuring American spirituals, a suite from Jerome Kerns "Showboat," and a reprise of "We Need to Talk" from Fred Onovwerosuoke's "A Triptych of American Voices: A Cantata of the People" at the Hatch Shell on the Charles River Esplanade, Wednesday, July 31 at 7pm, rain date: Thursday, August 31