Geology, water resources and usable ground-water storage capacity of part of Solano County, California

Abstract

The area described is confined largely to the valley-floor and foothill lands of Solano County, which lies directly between Sacramento, the State capital, and San Francisco. The area is considered in two subareas: The Putah area, which extends from Putah Creek southward to the Montezuma Hills and from the foothills of the Coast Ranges eastward to the west edge of the Yolo Bypass; and the Suisun-Fairfield area, which is to the southwest in the notch in the Coast Ranges through which the waters of the Great Central Valley of California reach San Francisco Bay. There are no known hydrologic interconnections between the two subareas, through either surface streams or underground aquifers. The climate of the area is characterized by warm, rainless summers and by cool winters in which temperatures seldom drop much below freezing. The rainfall ranges from about 17 inches per year along the east side to perhaps 24 inches in the foothills to the west, and irrigation is necessary for all crops except dry-farmed grains, pastures, and some orchards. PUTAH AREA The Putah area occupies the southwestern corner of the Sacramento Valley, a topographic and structural basin underlain by a thick accumulation of sediments eroded from the surrounding hills and mountains by the Sacramento River and its tributaries. The eastern Coast Ranges and foothills lying west of the Sacramento Valley are a generally northward-trending belt of eastward-dipping sedimentary rocks that range in age from Cretaceous to Pleistocene. Successively younger strata are exposed eastward, and the essentially undeformed deposits of late Pleistocene and Recent age that immediately underlie the valley lap onto the tilted sediments of the foothills. Most of the streams of the Putah area rise east of the high ridge of Cretaceous rocks marking the western boundaries of Solano and Yolo Counties, but Putah Creek, the largest stream in the area, rises far west of that ridge and flows across it in a deep, narrow canyon. Putah Creek and the smaller streams have constructed an alluvial plain, herein designated the Putah plain, which slopes eastward and southeastward from the foothills toward the Sacramento River. A large part of the Putah plain is traversed by a branching set of distributary channel ridges or natural levees formed at times of overflow of Putah Creek. The rocks in the Putah area range in age from Cretaceous to Recent. For the purposes of this investigation they are divided into eight geologic or stratigraphic units, from youngest to oldest: (1) Stream-channel deposits, (2) younger alluvium, (3) older alluvium, (4) Tehama formation and related continental sediments, (5) volcanic sedimentary rocks, (6) basalt, (7) undifferentiated sedimentary rocks of Paleocene(?) and Eocene age, and (8) undifferentiated rocks of Cretaceous age. The stream-channel deposits are predominantly loose sand and gravel along the channel of Putah Creek. In part they are actively moving downstream and shifting. The younger alluvium, of Recent age, consists of flood-plain deposits underlying the Putah plain, Vaca Valley, Pleasants Valley, and the small valleys in the foothills north of Putah Creek and in the English Hills. Exposures of younger alluvium are characterized by soils lacking significant profile development and in many places by channel-ridge topography. The older alluvium occupies the stratigraphic interval between the younger alluvium and the Tehama formation and related continental sediments and is probably of late Pleistocene age. Its contact with the underlying Tehama formation and related continental sediments is unconformable near the foothills, but it may be gradational beneath much of the Putah plain. The base of the older alluvium is not well defined at many places but is inferred to be at the bottom of an irregular and ill-defined zone of coarse deposits, which ranges from about 50 feet to more than 150 feet below the land surface. Exposures of the older