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68.
<br />from ground water aquifers. Nonetheless, such monitoring is required. Only with the
<br />results of measurements at existing mines can projections be made for future mines.
<br />Considering than the probable importance of subirrigated alluvial valley floors,
<br />and the potential for disrupting the hydrologic and biologic functions thereof, it
<br />was considered appropriate to evaluate the amount of land that would be classified
<br />as subirrigated alluvial valley floors. This paper reports efforts directed at the
<br />identification of alluvial valley floors as those areas are defined in proposed
<br />national legislation. Appendix 2 provides quotations from legislative proposals and
<br />Committee Conference Reports that have addressed mining in alluvial valley floors.
<br />These proposals may be summarized as indicating that an alluvial valley floor is:
<br />- A geologic unit comprised of several parts: an active (as opposed to
<br />abandoned or buried) stream channel (perennial, intermittent, or ephemeral),
<br />a flood plain, and an adjacent low terrace, including alluvium with suffi-
<br />cient soil depth and suitable texture to support perennial grasses suit-
<br />able for grazing or other agricultural use, and with sufficient channel
<br />stability to allow agricultural use;
<br />- A hydrologic unit in which the natural surface water and ground water is
<br />adequate to support agricultural activities such as hay production, by
<br />virtue of sufficient moisture held available to enable continued growth
<br />of suitable vegetation during the dryer summer and fall months;
<br />- A topographically - defined area in which the floor plain and adjacent low
<br />terraces may be irrigated by simple spreading of ephemeral waters and /or
<br />by simple diversion of natural flow *, and
<br />- An area of land used for farming.
<br />This report includes both a reconnaissance identification of subirrigated allu-
<br />vial valley floors, through analysis of surface features, and a preliminary analysis
<br />of hypothetical subsurface conditions that appear representative of actual condi-
<br />tions. It was not possible to investigate the actual uses or productivity of those
<br />areas mapped as alluvial valley floors. In the context of proposed legislation, it
<br />is therefore inappropriate to directly transfer the areas of alluvial valley floors
<br />presented herein to an impact on surface - mineable coal since the proposed legisla-
<br />tion applies to farmed areas (and to water quantity and quality) and "farmed areas"
<br />are not identified. It is similarly inappropriate to conclude that this study has
<br />adequately documented the hydrologic and biological systems that exist in alluvial
<br />valley floors. We have, however, attempted to provide an analysis of specific im-
<br />pacts on the hydrologic system that may result from surface methods of mining shal-
<br />low coals. This study draws upon research performed by others to initiate an
<br />analysis of whether those functions of alluvial valley floors deemed critical in the
<br />study can be reestablished during mining and reclamation.
<br />Research to Data
<br />The concept of alluvial valley floors utilized recently is new to those not
<br />directly associated with western agriculture. Little research has been initiated on
<br />the geohydrologic and biologic inter - relationships within alluvial valley floors or
<br />on land use activities conducted therein. This section discusses pertinent investi-
<br />gations reported to date.
<br />* We assume that the practice of spreading flood waters includes simple furrows and
<br />temporary stream diversions, but excludes extensive grading of land and /or construc-
<br />tion of canals. These latter efforts may be used to convey water from streams and
<br />impoundments to high terraces covered with thin alluvium and colluvium and lying
<br />outside the subirrigated alluvial valleys.
<br />69.
<br />The United States Geological Survey of the U. S. Department of the Interior
<br />conducted a field reconnaissance mapping of alluvial valley floors in the three -
<br />county portion of the Powder River Basin of southeastern Montana in 1975 ( Malde and
<br />Boyles, 1976). In that study alluvial valley floors were defined as those low ter-
<br />race and flood plain areas of drainages whose vegetation, in terms of species and
<br />cover, reflected the near surface availability of water. Terraces generally not
<br />higher than 1.5 meters above the channel floor of small streams and not higher than
<br />2.5 meters above the channel floor of principal streams were included in the alluvial
<br />valley floor description. Areas obtaining all or the vast majority of water for
<br />vegetation from ditch irrigation were not knowingly included.
<br />Malde and Boyles had no direct ground water data justifying their criteria and
<br />had to depend on knowledge of geologic and biologic phenomena. The areas mapped as
<br />alluvial valley floors supported vegetation systems dominated by grasses and often
<br />mixed with silver sagebrush (Artemisia cana) along small streams and in the upper
<br />reaches of large streams. Greasewood (Sarcobatus) and big sagebrush (Artemisia tri-
<br />dentate) were absent or rare in the alluvial valley floors mapped by Melds and
<br />Boyles. Greasewood and big sagebrush were usually considered indicators of deeper
<br />ground water and thus to be. located outside the alluvial valley floor.
<br />Malde and Boyles found that: (1) larger alluvial valley floors were found along
<br />the larger streams of the study area; (2) alluvial valley floor width generally in-
<br />creases in the downstream direction; (3) alluvial valley floors are generally con-
<br />tinuous upstream from the mainatem of major rivers to the point where vegetative
<br />and terrace characteristics are lost; and (4) comparing the areas mapped with coal
<br />resource data, less than three percent of the area of strippable coal in the south-
<br />eastern Montana study area is overlain by alluvial valley floors. Thus the size of
<br />alluvial valley floors was found, in practice, to be a direct function of annual
<br />flows, but to be a relatively small area of the entire coal region in southeastern
<br />Montana. We have visited the area between Birney Day School and Decker, Montana,
<br />mapped by Malde and Boyles and, based on our agreement with their mapping boundaries,
<br />concur with their findings, though the vegetation and topographic criteria may not
<br />be completely applicable for all climates and physiographic provinces in other areas
<br />of Montana and the interior western United States.
<br />Malde and Boyles' work is reconnaissance to the extent that no direct measure-
<br />ment of ground water levels, soil moisture, species composition, productivity, or
<br />land use was made. However, it is one of two published mappings of alluvial valley
<br />floors and provides a good estimate of the nature and extent of such areas in the
<br />Powder River Basin of the Northern Great Plains. In the recent past, their efforts
<br />have been further confirmed by detailed field mapping of alluvial deposits by the
<br />U. S. Geological Survey ( Malde, personal communication, 1976).
<br />A similar field reconnaissance study and mapping was recently conducted by the
<br />Water Quality Bureau, Montana Department of Health and Environmental Sciences in
<br />Dawson, Richland, and Garfield Counties in the Williston Basin of northeastern Mon-
<br />tana (Schmidt, 1977). This area, underlain by strippable lignite deposits, has a
<br />different physiography, climate, and land use pattern than the area studied by Malde
<br />and Boyles. Notable differences include the presence of glacial till and glacial
<br />landforms, the occurrence of many saline soils, the existence of larger deposits of
<br />gravel which outcrop in valley areas, and recent changes in land use for wheat and
<br />bay production. The major difference was the inclusion of somewhat higher terraces
<br />in the definition of alluvial valley floors since field surveys gave evidence of
<br />subirrigation at higher elevations above the stream channel than were found by Malde
<br />and Boyles. Schmidt found significant concentrations of silver sagebrush outside
<br />the alluvial valley floor areas. These differences illustrate the regional diversity
<br />of alluvial valley floors due to climate, hydrology, and physiography and demonstrate
<br />the need for field assessments of alluvial valley floors if one is to be certain of
<br />their nature and of their role in local land use patterns.
<br />More specifically, Schmidt found vegetative evidence of shallow ground water on
<br />terraces 2.5 meters above the stream channel along small streams and on three -meter
<br />
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