The D&L and Delaware Canal Towpaths are quite flat, as you might expect from trails that follow alongside rivers.
Heading west to east, there is a gradual descent from 235 feet above sea level at Allentown, to 163 feet at Easton, to about 20 feet at Bristol. Spread out over 75 miles, this is such a gradual change that most riders cannot feel it, regardless of which direction they are traveling.
Trail surface: the solid portions of the chart below represent paved surfaces, and the striped sections are non-paved.