2012-06-20_PERMIT FILE - C2010089 (88)

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introduction 61. SUBIRRIGATED ALLUVIAL VALLEY FLOORS - A Reconnaissance of Their Properties and Occurrence on Coal Resource Lands in The Interior Western United States* March 1977 John E. Hardaway Chief, Research & Applications Branch Dan B. Kimball Environmental Protection Technologist Shirley F. Lindsay Terrestrial Biologist Jack Schmidt, Larry Erickson Consulting Geologists U. S. Environmental Protection Agency, Region VIII Denver, Colorado Concern for Alluvial Valley Floors and the Impacts of Surface Coal Mining Concern has been expressed regarding the long -term impacts of surface coal mining on lands in the western United States currently or potentially useful for agriculture. Part of this concern bears on disruption of these lands used by wildlife. In particu- lar, there are questions regarding the potential impact of coal extraction in certain lowland areas of the semi -arid West where shallow ground water and /or soil moisture is adequate to support growth of grasses and forbs through the dry months. These land areas, located along drainage channels and referred to recently as "alluvial valley floors" (National Academy of Sciences, 1974), are most important in semi -arid and arid climates because water is "stored" in the alluvium, enabling vegetation to continue growth during the months of low rainfall. Apparently,'soil moisture, and perhaps some near - surface ground water, subirrigates the vegetation. The character of the vegetation is a function of both depth to water table (depth to saturated zone) and quality of the soil moisture and ground water. The depth to water table, as well as seasonal variations in this parameter, may be criti- cal in determining the composition of the vegetation. For example, if the saturated zone is sufficiently near the ground surface to permit capillary migration of ground water to the surface, subsequent evaporation may cause accumulation of salts. Under such conditions, the vegetation may be comprised principally of salt- tolerant species, such as alkali sacaton (Sporobolus airoides). Similarly, the configuration and moisture retention characteristics of the capillary and remaining unsaturated zone may be the most critical elements of the ground water system that supports vegeta- tive communities in the lowland areas. However, no one has yet reported on the essential functions critical to sustained growth of vegetation in these areas. Alluvial valley floors are used for grazing and for production of hay. Both domesticated and wild animals are attracted by the more vigorous vegetative growth and the presence of surface water in these lowland areas. The alluvial valley floor areas include the principal surface water accumulation points, as well as points of ground water recharge and discharge. The composition of vegetation in subirrigated alluvial valley floors is import- ant since some grass and legume species are more desirable as hay. Some vegetative * This article is based on a draft report being developed by the authors. However, no official support or endorsement by the Environmental Protection Agency or any other agency of the federal government is intended or should be inferred.