Geology and Ground Water Resources of the Matanuska Valley Agricultural Area, Alaska.

نویسنده

  • F W Trainer
چکیده

This report describes the geology and ground-water resources of an area of present and potential agricultural development in south-central Alaska. The Matanuska Valley agricultural area, which covers about 350 square miles, is in a wide valley flanked by rugged mountains. The valley floor is underlain nearly everywhere by glacial drift whose total thickness is known at relatively few places. Bedrock is exposed or known to be near the surface in only a small part of the entire valley floor. Nonglacial unconsolidated deposits include windblown material distributed generally over the agricultural area and slope deposits along the valley walls. The youngest drift, which is at the land surface throughout the area, is thought to be of late Wisconsin age. Exposures and well logs record at least one older drift sheet in several localities, and two older drifts are known to be present at one place in the agricultural area. These older deposits have no topographic expression in the valley floor. The drift includes till, outwash stream deposits, and estuarine and lake (?) deposits. Physiographic features formed by these deposits in or adjacent to the agricultural area include an end moraine, lateral moraines, eskers, crevasse fillings and other pitted features, river terraces, outwash flood plains, and an extensive estuarine flat. Stagnation of the ice was an important phase of the deglaciation of this area. The topographic form of the valley floor is due chiefly to deposition by glacial ice and melt water, iceblock pitting, and terracing by melt-water streams. The till, known locally as "hardpan," is characteristically silty or clayey, tough, and relatively impermeable. The youngest till, which is the best known, forms ground moraine in part of the area and a buried deposit in much of the remainder of it. From information obtained in the relatively small parts of the area where wells are closely spaced the till appears to be a single, more or less continuous, sheet. This till commonly ranges in thickness from about 10 to about 60 or 70 feet; several wells penetrate till 100 feet or more thick (in one well its thickness appears to exceed 400 feet), but it is thought that the thicker sections include two or more till units. The chief hydrologic significance of the till is that it forms a confining layer for artesian aquifers. The till is generally not water bearing except where it contains layers of sand or gravel, and these are commonly thin and yield only small quantities of water. The outwash deposits are chiefly sand and sandy gravel. Boulder gravel is present in some places, especially in former drainage courses that were probably near the melting ice. The well-sorted materials are relatively permeable and transmit water readily. Outwash deposits of silty sand and 2 GEOLOGY AND GROUND WATER, MATANUSKA VALLEY, ALASKA poorly sorted silty gravel are much less permeable. Individual sandy and gravelly beds in the outwash deposits are commonly thin and interlayered. However, a number of wells have penetrated thick deposits of sand; much of this sand becomes unstable under the differences in hydrostatic head commonly produced during drilling or pumping and flows into the wells as "quicksand." In much of the area, sheetlike outwash deposits just beneath the land surface range in thickness from a few feet to more than 100 feet. Ground water in these deposits is unconfined. In some places perched or semiperched ground water is present above till, bedrock, or layers of stream-laid silt. Other outwash deposits are buried beneath till. They are known to be as much as 50 to 60 feet thick and probably are considerably thicker in some places. They commonly contain confined (artesian) ground water. Well logs and hydrologic data suggest that buried outwash deposits form a continuous or almost continuous layer in an area of more than 10 square miles near the community of Palmer. Similar buried deposits are known to be present in several other parts of the agricultural area also. The area was overridden by two large valley glaciers that joined here to form a piedmont ice lobe. The repeated advance and recession of the ice, the effects of melt-water streams, and possibly the formation of temporary lakes contributed to the complex stratigraphy of the drift which makes prediction of the presence and character of aquifers difficult. Ground water occurs in the mantle of windblown material (loess and sand) only under special conditions, but the mantle is important hydrologically because it absorbs precipitation readily. This absorption provides soil moisture, leads to ground-water recharge, and reduces direct runoff. The bedrock, chiefly sandstone, shale, and greenstone of Cretaceous and Tertiary age, is not an important aquifer. Most wells in the agricultural area tap sand and gravel of the outwash deposits, and household and farm wells tapping suitable material generally provide dependable supplies. Only a few larger capacity wells have been constructed. Two wells belonging to the city of Palmer have produced an average of about 100,000 gallons per day (gpd) since late 1953, and waterlevel records suggest that equilibrium of recharge to and discharge from the aquifer has been attained near these wells for this rate of withdrawal. Data provided by test pumping two wells in other parts of the agricultural area suggest that properly constructed wells, penetrating a sufficient thickness of favorable water-bearing material, may produce as much as 100 to 200 or more gallons per minute. However, no information is available regarding the effect of irregularities of the drift stratigraphy near those wells on the maintenance of sustained yields over long periods. Except for the relatively heavy pumping of the two municipal wells, ground-water withdrawal in the agricultural area has been on such a small scale and so widely dispersed that it probably has had a negligible effect on ground-water levels. In the development of ground-water supplies in this area, the chief problems that cannot be solved by improved well construction are thought to be due to the apparent absence of suitable water-bearing material in places where sand becomes "quick" during the drilling or where little or no permeable material is penetrated. Replenishment of the ground water is chiefly from precipitation. However, probably only a small proportion of the annual precipitation, which averages INTRODUCTION O about 15 inches, reaches the ground-water body, and very dry seasons are accompanied or followed by a marked decline of water levels in some wells. In a few places water-table aquifers are recharged by water from streams. Natural discharge from the aquifers occurs by seepage and spring flow into streams and lakes, by evaporation, and by transpiration by plants. The ground water is a moderately hard, calcium magnesium bicarbonate water generally suitable for domestic purposes. A few wells have obtained salty water or water that has objectionable hardness, iron content, or other characteristics. The salt water is thought to have been trapped in the bedrock when marine or estuarine water lay over this part of the region. The area is divided into six physiographic units to facilitate description of the occurrence of ground water. Data include records of 391 wells, whose locations are shown on the geologic map, the logs of 44 wells, and chemical analyses of 27 ground-water samples. INTRODUCTION LOCATION AND EXTENT OF AKEA The Matanuska Valley is a part of the lowland lying north of the Chugach Mountains in south-central Alaska (fig. 1 and pi. 1). The valley of the Matanuska River and the lowland extending westward from it to the Susitna River are in the Matanuska and Wasilla districts as denned by P. S. Smith (1939, pi. 3). The area described in the present report (fig. 1), hereafter termed "the Matanuska Valley agricultural area," is best known as the site of agricultural colonization undertaken by the Federal Government in 1935. It is bounded on the north by the Little Susitna River and, east of the mountain canyon of that stream, by the Talkeetna Mountains. On the south it is bounded by the Knik River and Knik Arm. It extends from Eska Creek on the northeast to Goose Bay on the southwest. As thus defined the area lies approximately between 148° 55' and 149° 50' west longitude and between 61°25' and 61°45' north latitude. It covers about 350 square miles. PURPOSE AND SCOPE OF INVESTIGATION The field study on which this report is based was made by the writer during the period 1949-55 as part of the investigation of the ground-water resources of Alaska begun by the U.S. Geological Survey in 1947. The purpose of this investigation in the Matanuska Valley was to map the water-bearing materials and to determine the occurrence, availability, and quality of ground water in the area. The need for the compilation and interpretation of geologic and hydrologic data became important 4 GEOLOGY AND GROUND WATER, MATANUSKA VALLEY, ALASKA with the colonization in 1935; this need has increased during the postwar period of continuing development. Most of the inhabitants depend upon wells for their water supply, and those settling in undeveloped areas have lacked information on the availability of ground water. More extensive utilization of ground water, possibly including irrigation, undoubtedly will come in the future. Geologic features were mapped on aerial photographs, and the data were later transferred to a base map. The base used was taken from parts of the Sutton, Matanuska, Eklutna, Houston, and Knik sheets of the U.S. Army Map Service. Some mapping was done also on the more recent Anchorage B-8, C-6, C-7, and C-8 quadrangles of the U.S. Geological Survey. An inventory of wells was made, and a series of periodic observations of water levels in selected wells was continued throughout the field study. Seven test holes were drilled by jet-percuscussion and cable-tool methods. Four quantitative tests of aquifer characteristics were made by pumping wells under controlled conditions. Cuttings from wells and samples of unconsolidated sediment from outcrops were examined to determine their texture and composition. The permeability of several small undisturbed samples was determined in the field with a variablehead permeameter (Wenzel, 1942, p. 64). Samples of water were collected from representative wells for chemical analysis. Data from 391 wells are given in table 5; the locations of the wells are shown on plate 1. In addition, drillers' logs of 44 wells are given in table 4, and chemical analyses of 27 water samples in table 3. The investigation was made under the supervision of D. J. Cederstrom, district geologist for Alaska. M. J. Slaughter, R. M. Waller, and G. W. Whetstone assisted in certain phases of the fieldwork. Leonard Reynolds, George Ramsey, and Glenn Ramsey drilled the test wells constructed as part of the investigation. E. C. Casey, D. A. Morris, D. C. Phillips, Clifford Shaw, M. J. Slaughter, R. M. Waller, and G. W. Whetstone made many water-level measurements, and Mr. Waller determined the altitudes of several wells by instrumental leveling. PREVIOUS INVESTIGATIONS The geology of the Matanuska Valley agricultural area or of parts of it has been described in several reports. Capps (1940) discussed the geology of the general region. Martin and Katz (1912) described the part of the area near Moose and Eska INTRODUCTION 5 Creeks, and Landes (1927) described the district between the Knik and Matanuska Rivers. Rockie (1946) described the physical geography of the agricultural area in a report on soils and land conditions. The writer (1953) has given a preliminary description of the geology and ground-water resources. Other papers, including those by Black (1951), Karlstrom (1952, 1953, 1955b), Kellogg and Nygard (1951), Martin (1942), Pewe and others (1953, p. 12-13), Rockie (1942), Stump, Handy, Davidson, and Roy (1956), Stump, Handy, Davidson, Roy, and Thomas (1956), Stump and Roy (1956), and Tuck (1938), have been devoted to special problems relating to the geology of the area. ACKNOWLEDGMENTS For many courtesies the writer is indebted to Mr. C. W. Wilson and his staff, of the Soil Conservation Service, Palmer, Alaska; to Messrs. D. L. Irwin, A. H. Mick, and staff, Alaska Agricultural Experiment Station, Palmer; and to officials of the city of Palmer. The Alaska Road Commission permitted access to highway rights-of-way for test drilling. Mr. R. T. Mathews, Alaska Public Works, Anchorage, made available facilities of the Palmer water system, then under construction, for pumping tests. Mr. James Hurley gave the writer copies of well logs from the files of the Alaska Rural Rehabilitation Corporation. The Matanuska Valley Fair Association permitted the use of storage space. Special thanks are due the late Professors Kirk Bryan and H. C. Stetson, and Professors M. P. Billings, K. F. Mather, and J. P. Miller, of Harvard University, and C. E. Stearns, of Tufts University, for their discussion of and many suggestions regarding the writer's work. Without exception, residents of the area permitted access to wells on their property or provided information regarding them. Messrs. Samuel Gotten, James and Albert Frey, Henry LaRose, A. R. Moffitt, and Thomas Moffitt, who have drilled many wells in the agricultural area, described their experience and gave the writer much valuable information. The cooperation of Messrs. Glen Woods, Henry Liebing, and Loren McKechnie facilitated the construction and testing of test holes. The owners of the observation wells listed permitted use of their well records, and Messrs. J. C. Baldwin, Ted Buzby, Henry LaRose, F. B. Linn, Loren McKechnie, G. E. Murphy, Oscar Tryck, Noel Woods, and the personnel of the Alaska Agricultural Experiment Station made many periodic water-level measurements. 6 GEOLOGY AND GROUND WATER, MATANUSKA VALLEY, ALASKA GEOGRAPHY CLIMATE The climate of the eastern part of the Cook Inlet lowland, which includes the Matanuska Valley agricultural area, is the result of a combination of marine and continental influences. Near the ocean but separated from it by the Chugach Mountains, the lowland lacks both the high precipitation of areas bordering the Gulf of Alaska and the temperature extremes of the interior of the Territory. Dale (1956) has described the climate of the Matanuska Valley. Climatological data have been collected at several localities in the agricultural area in recent years. The longest record is that for the Alaska Agricultural Experiment Station near Matanuska. Selected data for this locality are presented in table 1. For some years of record the total precipitation in the Matanuska Valley has been as much as one-third greater or less than the mean. The mean total seasonal snowfall is nearly 4 feet, but the annual departure from the mean may be as great as half this amount. In most years the winter and spring are relatively dry, and about two-thirds of the annual precipitation occurs during the 5-month period June to October. Wide departures from the mean temperature are well illustrated by variations in the length of the growing season. The last spring frost commonly occurs in late May, and the earliest autumn frost, in late August or September. During a total of 35 years for which records are available, the length of the growing season averaged 106 days but ranged from 59 to 140 days. Midsummer temperatures in the agricultural area commonly range from 45° to 70° F; temperatures as high as 80° F are unusual. The winters are moderately cold, but periods during which the temperature reaches 20° to 30° F are usually short. The autumn freezeup comes in October or November, and seasonal frost commonly reaches depths of 6 feet or more. The ground begins to thaw in April or May, but seasonal frost may persist beneath the surface in protected spots as late as July. Appreciable microclimatic variation occurs within the agricultural area, perhaps because of differences in topography or because of other influences. The geographic distribution of light summer showers and the geographic variations in the occurrence of frosts in spring and autumn are particularly noticeTA BL E 1 . C li m at ol og ic al d at a fo r A la sk a A gr ic ul tu ra l E xp er im en t St at io n, n ea r M at an us ka , 19 21 -5 2* P re ci pi ta ti on , in i nc he s: M ea n_ __ __ __ __ __ __ __ __ _ M in im um __ __ __ _ __ __ __ T em pe ra tu re , in ° F M ea n m on th ly __ __ __ __ __ _ Ja n. 0. 99 2. 89 .2 6 13 .1 21 .4 3 .7 Fe b. 0. 68 3. 16 .1 0 18 .1 27 .3 9 .3 M ar . 0. 52 1. 42 .0 0 25 .0 33 .6 15 .1 A pr . 0. 42 1. 64 .0 0 36 .5 45 .5 26 .9 M ay 0. 66 2. 31 (2 ) 46 .9 57 .8 35 .7 Ju ne 1. 34 4. 62 .1 6 55 .2 66 .4 43 .7 Ju ly 1. 97 3. 75 .5 5 57 .6 67 .7 47 .4 A ug . 2. 92 6. 37 .4 5 55 .5 64 .9 45 .9 Se pt . 2. 70 7. 55 .5 1 47 .8 56 .7 38 .5 O ct . 1. 80 4. 61 .3 9 36 .6 44 .0 28 .5 No v. 0. 97 3. 71 .0 4 23 .4 30 .0 15 .0 D ec . 0. 99 3. 81 .0 4 14 .3 21 .5 5 .5 A nn ua l 15 .9 6 35 .8 1 D at a fr om U .S . W ea th er B ur ea u. 2L es s th an 0 .1 i nc h. 8 GEOLOGY AND GROUND WATER, MATANUSKA VALLEY, ALASKA able. For example, the average length of the growing season at Weather Bureau station Palmer IN, at Palmer (about 6 miles northeast of the Experiment Station) was 26 days longer than that at the Experiment Station during the period 1942-55. The seasonal distribution of rainfall (little rain early in the growing season), the wide departure from the mean precipitation during many seasons, and the late spring and early autumn frosts during many years are responsible for a measure of uncertainty in crop yields in the agricultural area. The dominant wind of this region, known locally as the "Matanuska wind," is from the northeast. It is an autumn and winter wind and sometimes blows almost continuously for several days or longer. Weather Bureau records indicate that gusts reaching 50 or more miles per hour occur during the more severe storms. The "Knik wind," produced by the flow of oceanic air from the Gulf of Alaska moving down the Knik Valley, is relatively warm. During late winter and spring it brings mild weather and, in many years, removes much of the snow cover from the agricultural area before the ground begins to thaw. TOPOGRAPHY AND DRAINAGE The Matanuska Valley agricultural area lies in a wide flatfloored valley formed by the merging of the Matanuska and Knik valleys at the eastern end of Knik Arm (fig. 1). The valley is bounded by rugged mountains which rise abruptly above its floor. In the Chugach Mountains, at the southern edge of the valley, Pioneer Peak rises to an altitude greater than 6,300 feet; several other peaks surpass 4,000 feet; and altitudes of 3,000 feet are common. Along the northern edge of the valley, peaks in the Talkeetna Mountains reach altitudes of 3,000 to 5,000 feet. Although the altitude of the valley floor ranges from tide level at Knik Arm to 1,000 feet at the base of Wishbone Hill, the local relief is commonly not more than 100 to 200 feet. (See fig. 1 for the locations of geographic features named.) Bodenburg Butte, which is almost 800 feet higher than the surrounding lowland, and other similar hills of rock provide greater relief. The bluffs along the Matanuska River north of Palmer rise 200 to 300 feet above the river flood plain. Most of the valley floor is a gently rolling surface crossed by narrow flat-floored stream courses. The hills and intervening valleys commonly trend southwestward. This 'characteristic is shown most conspicuously by two linear series of lakes near Wasilla, but is repeated by many smaller features. Exceptions

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عنوان ژورنال:
  • Science

دوره 119 3089  شماره 

صفحات  -

تاریخ انتشار 1954