Q: Two friends at my weekly 42 dominoes game are from the Rio Grande Valley. When I asked them why Texans call a flat floodplain a “valley,”when a valley is actually the low spot between hills or mountains, their answer was, more or less, “It’s a valley.” Is it, though?Tim Ziegler, AustinA: The southernmost portion of Texas has, over the years, been known by many a moniker, among them the Rio Grande Valley, the Lower Rio Grande Valley, the Valley, El Valle, the RGV, and the 956, which is, of course, the area code for most of the region.A valley, however, this area is not, at least in a strictly topographical sense. If you had to give the place a more accurate name, you’d probably…