Complex sentence: Between 1911 and 1922, the Potomac River was repeatedly dredged to deepen the channel and to widen the distance between Analostan/Theodore Roosevelt Island and Columbia Island (so that the "Virginia Channel" west of Analostan/Roosevelt Island would not flood easily).
Split into simpler sentences: Between 1911 and 1922, the Potomac River was repeatedly dredged to deepen the channel and alleviate flooding. Dredging widened the distance between Analostan/Theodore Roosevelt Island and Columbia Island (so that the "Virginia Channel" west of Analostan/Roosevelt Island would not flood easily).