Scoggins Dam is a 151-foot-high zoned earthfill structure that is 2,700 feet long at the crest and contains 4 million cubic yards of material. The upstream side of the dam is faced with rock riprap for protection against wave action; the downstream side is faced with topsoil and planted with grass. Total capacity of Henry Hagg Lake is 59,910 acre-feet (active 53,600 acre-feet).
As part of the operation and maintenance modification activities, a drain was installed on the left abutment in 1987.
Scoggins Dam and Reservoir are situated in the foothill on the eastern slope of the north-trending coast ranges of northwestern Oregon. This portion of the eastern flank of the coast ranges is a northeast-dipping monocline consisting of marine volcanic and sedimentary formations of Tertiary age. Near the eastern limit of the foothills (east of the damsite), these units are unconformably overlain by the Columbia River Basalt of Miocene age. The basalt is in turn unconformably overlain by various nonmarine sedimentary units of Pliocene and Pleistocene age (Schlicker and Deacon, 1967; fig. EG-1). This region is broken by a series of northwest trending faults having displacements of several hundred to more than a thousand feet (Schlicker and Deacon, 1967). All major valleys, including Scoggins Creek on which the dam is situated, are underlain by unconsolidated stream alluvium of recent (Holocene) age.
Stratigraphy. - The regional geologic units are described in detail below in order of decreasing age:
(1) Eocene volcanics and sediments undifferentiated - This unit is a sequence of structurally complex basalt flows, pillow lava, tuff, agglomerate, and breccia with well-indurated marine siltstone and sandstone interbeds. The sequence dips east and northeast and is several thousand feet thick in the Scoggins Creek area, thickening westward in the coast ranges. Only the extreme northwest portion of the reservoir is underlain by this unit. It is overlain, presumably conformably, by the Yamhill Formation.
(2) Yamhill Formation - The Yamhill Formation consists primarily of well-indurated, thin-bedded shale and siltstone with occasional interbeds of green basaltic sandstone and poorly sorted tuffaceous sandstone (photo EG-1). Locally, basalt and gabbro dikes and sills have invaded the formation. The Yamhill Formation underlies most of the Yamhill River Valley and trends northerly in a thinning outcrop belt to within a short distance north of Scoggins Creek. It is also present in isolated exposures in the bed of Gales Creek. Its thickness ranges from probably less than 1,000 feet in the Scoggins Creek area to more than 2,000 feet in the Yamhill River Valley. Its primary structure is monoclinal eastward with local irregularities. This unit weathers deeply and, because of its fine-grained nature, is very susceptible to landsliding on relatively shallow slopes (Tengesdal, 1968). The Yamhill Formation is lower upper Eocene in age and is unconformably overlain by the Spencer Formation.
(3) Spencer Formation - The Spencer Formation is exposed in a nearly continuous, 17-mile belt that commences just east of Carlton and trends northwesterly nearly to Gales Peak, a few miles north of Scoggins Creek. The formation is more than a mile wide east of Carlton and narrows to about a quarter of a mile where it crosses the Tualatin River and Scoggins Creek. The formation consists of thick-bedded to massive, well-sorted, friable, fine- to medium grained feldspathic sandstone with occasional thin carbonaceous siltstone and claystone interbeds. In the top and bottom portions of the formation, thin-bedded siltstone and claystone predominate. The sandstone is typically composed of about 40 percent quartz, 55 percent plagioclase feldspar, and 5 percent muscovite, biotite, and chlorite. In the northwest, two-thirds of its outcrop belt, where it averages 200 feet in thickness, the formation is composed almost entirely of friable, fine sandstone. This unit is less susceptible to landsliding than the other Tertiary marine units in the area. The Spencer Formation is upper Eocene in age and is overlain by Oligocene marine sediments.
(4) Oligocene marine sediments undifferentiated - The uppermost rocks of the Oligocene sequence are composed of tuffaceous sandstone and siltstone. Beneath this sequence, in most areas, is a section of moderately indurated quartzitic sandstone' The lower part of the Oligocene section generally consists of siltstone, basaltic sandstone, and local conglomerate. In the Scoggins Creek Quarry, basaltic sandstone and conglomerate occur below a well-indurated, limey sandstone which contains abundant megafossils in several 1-foot layers. The total maximum thickness of the undifferentiated Oligocene sequence is estimated to be about 3,000 feet, although its thickness varies considerably in the map area. On the basis of faunal data, the lower Oligocene section is estimated to be about 1,000 feet and the upper Oligocene about 2,000 feet thick. The Oligocene sequence generally dips to the east and northeast at low to moderate angles. Locally, fault beds dip 40 to 500. In a few areas, dips are reversed to the south and southwest to form local anticlines. This sequence is overlain unconformably by the Columbia River Basalt.
|(5) Columbia River Basalt - The Columbia River Basalt is widespread, is the bedrock of many of the hills, and underlies most of the valleys east of the damsite. The formation is composed of a series of weathered and unweathered lava flows with scattered interflow zones of breccia, ash, and baked soil horizons. This formation is Miocene in age and does not exist in the dam or reservoir area.|
|(6) Willamette Silt - The Willamette Silt underlies nearly all of the lowlands east of the damsite. It generally extends onto the surrounding uplands to an approximate average elevation of 250 feet, where it occurs on sloping terraces. The Willamette Silt lies on the erosional surfaces of all the older bedrock units. The unit is composed of unconsolidated beds and lenses of fine sand, silt, and clay. Stratification is commonly on the order of 4- to 6-inch beds; 3- to 4-foot beds are locally present; and in many areas, the silt is massive with indistinct stratification. Lenses of pebbly, fine to medium sand with scattered cobbles of granite and quartzite occur in some of the outcrops. The silt is usually light brown to buff in color and occasionally light gray where granular soils predominate. This unit is upper Pleistocene in age and does not exist in the dam or reservoir area.|
|(7) Young alluvium - Young alluvium is present in the flood plains, channels of all of the main streams, and most of the smaller tributaries in the region. The composition of the young alluvium is mainly silty clay, clayey silt, and fine sand, with local areas of peat and organic clay. The nature of the deposits is dependent upon the rock types in the drainage source area and upon local groundwater conditions. This unit is recent (Holocene) in age.|
|Structure - The region surrounding the dam and reservoir site is structurally simple. All pre-Quaternary units are part of a large homoclinal structure on the northeast flank of the coast ranges and have relatively gentle east to northeast dips. Locally, the area is cut by several northwest trending faults that appear to displace only Tertiary-age units (fig. EG-1). Little is known of the nature of these faults, although Schlicker and Deacon (1967) indicate vertical displacement on them. A fault zone is mentioned in the Geologic Report for Final Design (1969) that may correspond to the fault in Scoggins Creek. This fault dips 450 to the northeast, but sense of displacement was not determined.|
General - Scoggins Creek meanders southeasterly through the damsite area in a 1,700-foot-wide, flat-bottomed valley. The creek channel is about 30 to 40 feet wide and has been cut 10 to 15 feet into the valley fill near the right center of the valley floor. The abutments are on topographic spurs on the lower slopes of dissected ridges that parallel both sides of Scoggins Creek Valley. The slope above the left abutment rises on an average grade of about 12 percent, some 780 feet in elevation above the valley floor, to the crest of a ridge. The slope above the right abutment rises on an average grade of about 18 percent, some 540 feet in elevation above the valley floor, to the crest of a ridge. There are several large ancient landslides on the right side of the valley both upstream and downstream from the damsite and a few smaller landslides on the left side of the valley upstream from the damsite.
Bedrock units at the damsite and within the reservoir area, in order of decreasing age, are the middle Eocene sedimentary and volcanic undifferentiated unit in the upper reservoir area; siltstone and shale of the upper Eocene Yamhill Formation in the middle and lower reservoir area; and sandstone of the upper Eocene Spencer Formation at the damsite. All of these units are overlain by thin to thick accumulations of residual soil and slope wash, consisting of clayey soil on the valley sides, and alluvium in the valley bottom consisting of predominantly clay deposits with a few silt, sand, and gravel layers.
Bedding planes and contacts in the bedrock units strike north to northwest and dip relatively uniformly to the east and southeast between 10 and 20% Locally, bedding attitudes may vary from the regional trend due to landsliding and surficial creep.