Figure 4.25 Progression of thermokarst from 1949 to 2013 along a section of the Prudhoe Bay Spine Road. The Spine Road was constructed in 1969 so it does not appear on the 1949 image. Thin cloud cover obscures the small lake in the upper left corner of the photo, but most of the thermokarst pits that were present in 1972 are also visible on the 1949 image. In 1972, gravel from road construction occurs on both sides of the road. In 1979, some roadside thermokarst is visible near the road on the north side (above the road in the photo). By 2010, extensive thermokarst is present on both sides of the road, but is most developed south of the road due to periodic flooding from a nearby lake. By 2013, most thermokarst on the south side of the road was connected to Lake Colleen by continuous channels of water in the polygon troughs (Walker et al., 2014).