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Biological Soil Crust, N. Herbert NPS |
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Geology and slope strongly influence most Grand Canyon soils. Currently, soils throughout the Canyon are categorized as poorly developed. Soils are highly variable, ranging from moist forest soils of the North Rim to shallow, dry mineral soils and bedrock exposures of the inner canyon. Inner canyon soil textures are sandy loam, sands, or loamy sands. It is likely that there are a few silt loams or clay loams in the Hermit and Bright Angel shales and in the Toroweap Valley.
Most soil types in Grand Canyon erode very easily and regenerate slowly. Their sandy nature allows water to be absorbed immediately, leaving the ground dry shortly after rain showers. The soils are typically fragile and require little disturbance to create erosional problems. Large park areas show essentially no human impacts to soils. Other areas, which were used for farming, grazing and mineral extraction, as well as developed areas, have heavily impacted soils. Biological soil (“cryptogamic”) crusts are very sensitive soil systems, specific to arid lands. These crusts cover a significant portion of inner canyon soil. Cyanobacteria form the crust, while other bacteria, algae, fungi, lichens, and mosses are often present. These crusts play important roles in reducing soil erosion, increasing water conservation and in promoting nitrogen fixation. They create a more favorable environment for vascular plants to germinate under arid conditions. These crusts are highly susceptible to trampling and air pollution.
Soil surveys exist for about 23% of the park (188,000 acres on the Sanup Plateau and 93,500 acres on the Havasupai Traditional Use Lands). These areas were surveyed as part of grazing allotment analyses. However, extensive soil surveys have not been conducted for much of the Canyon. The soils along the Colorado River are known in more detail. Land areas along the river are characterized by fine-grained beaches, coarse-grained cobble bars, and tributary fan deposits. The fine-grained deposits found on river terraces may be classified according their age (pre- or post-Glen Canyon Dam), how they were deposited (floods, wind action, or underwater reworking below present high water) and soil grain size (cohesive silts, and sands with negligible silt).
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