From the Rio Grande to the Mississippi, the coast of Texas has a pleasing arch with plentiful barrier islands …and lagoons, coastal marshes and swamps. Inland 100 miles, however, the flat plains and lazy river valleys climb into a rolling, dissected landscape. Local relief and choppiness are quite uniform over a wide area. High densities of mid-20th century reservoirs spread over east Texas, right of the line between Houston and Austin. The Edwards Plateau and Ouachita Mountains frame the top with different terrain.