At 7.3 miles from New Boston, the volume of Deer River increases when a major tributary (Cobbs Creek) enters on river right. This marks the end of the gorge and the start of "The Flats." This section of the river is reminiscent of the tamer sections of the Salmon River in Pulaski.
At 11.7 miles (or 4.4 miles from Cobbs Creek) the river enters the town of Copenhagen and runs under the town bridge. Immediately past the Route 12 bridge is a series of class 3 ledges.