Summer air temperatures and the presence of permafrost in the subsoil of the Kobuk Valley create conditions suitable for the seasonal development of poorly drained, sparsely vegetated, swampy plains often referred to as muskegs or tundra. In our expedition down the Kobuk River, observations of muskeg were limited to the reach of the river within the broad, flat Kobuk Valley. Here, elevated above the banks of the Kobuk River, existed flat stretches of treeless plains interdispersed with swampy areas and tundra ponds extending from the river to the mountains encompassing the valley.
During the course of our eleventh day on the river, we finally reached a section of the river that had no vegetated corridor separating the river from the tundra. We were then able to scale the river bank and get our first look at the tundra. The landscape of the tundra consisted of areas of low lying plants and barren pockets of rocky soil. The low lying vegetation included plants such as moss, lichen, tussock grasses and blueberry. The tussock grasses made travel across the tundra somewhat cumbersome. Tussock grasses grow in bunches which form a shape similar to a mushroom, and as an individual steps on the bunch of grass, the small bottom portion of the bunch gives way. The barren portions of the tundra contained rocky soil with gravel to sand sized grains which contain little to no vegetation or organic material. Expedition members concluded that the rocky soil moved to the surface through a process called frost heave, which is the mixing of the soil column by the expansion and contraction of water molecules as they freeze and thaw each year.
Expedition members excavated a small soil pit in order to make observations about the subsurface of the tundra. Organic, peaty soils about 4-6 inches thick underlie the vegetation on the tundra. Beneath the organic layer the soil transition into what appeared to have been a windswept plain of glacial outwash. The glacial outwash was poorly sorted, moist and showed few signs of decomposition. The depth of the water table from the surface was about 12-18 inches. The temperature of the water groundwater measured with a thermometer was about 50 ºF. Although no permafrost was encountered while digging the soil pit, the lack of true soil horizonation and mineral decomposition in the coarse grained soil was evidence for a system dominated by permafrost.
Tundra ponds also supplied evidence of a permafrost dominated landscape. As seen in Figure 1, tundra ponds spotted the muskeg. In the tundra, ponded water fills in low areas formed by thermal karsting. Thermal karsting occurs when permafrost melts and internal drainage of the melt water causes the land surface to slump. At the time of our visit, it appeared that some of the ponds had recently been connected and water was most likely flowing in a down gradient direction towards the Kobuk River. There seemed to be a progression along the downward gradient of the state of the subsurface beneath the ponds from soft and mucky to more firm. One hypothesis to explain this observation is that the subsurface of the down gradient ponds had melted later in the season and more slowly than the up gradient ponds, and that the firmness of the substrate was due to the still frozen state of the soil.