Topography Napa Valley can be divided into a group of distinct topographical areas: the lowland Napa River valley between the
Mayacamas and
Vaca Ranges; the mountains themselves; and the intermontane, eastern portions of the county beyond the watershed of the Napa River. The elevational differences and relief between these areas are pronounced and influence all aspects of the region's physical geography (climate,
geomorphology,
hydrology, soils and vegetation). The floor of the Napa Valley is in length south to north and between wide. Traversing the entire length of the valley is the Napa River, which commences north of Calistoga and-drains into
San Pablo Bay. Along its course through the valley, the river elevation drops from around near the city of Calistoga to around near the city of Napa. The gently sloping valley floor, however, is interrupted by numerous
bedrock outcrops which form isolated
bench hills, hence, the Rutherford and Oakville Benches. In other places, the valley floor features broad alluvial fans extending toward the center of the valley from mountain streams which serve as
tributaries to the Napa River.The
Yountville Hills are the highest of these "bedrock islands" and have influenced the geographic evolution of the Oakville area. In other places, the valley floor features broad
alluvial fans extending toward the center of the valley from mountain streams which serve as tributaries to the Napa River. Two fundamental geographic distinctions within Napa Valley are particularly relevant to the delimitation of the Oakville viticultural area: On the east–west axis, mountain versus valley floor, delineating the valley floor viticultural environments; and on the north–south axis, climatic differences as the result of a decreasing incursion of maritime air into the valley. These distinctions can be integrated with the community identity of Oakville (and the other communities of Napa Valley) to provide consumers with meaningful and distinctive reference points concerning the viticulture of Napa Valley. From the perspective of a wine consumer, such basic geographic distinctions offer a useful introduction to the complexity of viticulture in Napa Valley.
Geology Geological history is an important factor in shaping Napa Valley viticultural environments. Napa Valley is largely a synclinal (down-folded) valley of
Cenozoic age.
Faulting (accompanied by minor
folding) throughout the valley later resulted in the formation of bedrock "islands" (outcrops) across the valley floor. These rock islands have been modified during the last million years through
erosion by the Napa River, its tributaries and other erosional slope processes. Sections of the old Napa River
channel are still visible here and there in the valley, including in several places within the Rutherford viticultural area. In this central portion of the valley, much of the old river channel and its alluvial
sediments have been buried by more recent Napa River floodplain sediments, but they principally have been covered by
alluvial fans emerging from the mountain streams on the western and eastern sides of the valley. The age and size of these fan surfaces are a function of climatic change, basin
lithology (mineral composition and structure of rocks), and basin size, all of which vary among the four major
drainage basins in the Rutherford and Oakville areas, accounting for differences in these fan surfaces. The northern fans (in the Rutherford area) are the larger geomorphic features, have more significantly controlled the course of the Napa River through time, and are geologically more diverse.
Climate A previously, published report, prepared by the
National Oceanic and Atmospheric Administration (NOAA) and submitted on behalf of the Napa Valley Appellation petition in 1980, established the general weather and climatic differences of Napa County. This report showed that Napa Valley can be divided into two general climatic regions (coastal and inland), and three topographical areas-the valley itself lying within the Mayacamas Range to the west and the Vaca Range to the east; the area within the mountains themselves; and the area covering the eastern portion of the county. The elevation within Napa County increases as one progresses north up the valley. With this increase in elevation there is an increase in precipitation, ranging from in the south to in the north. Additionally, the coastal influence in the Napa Valley results in a relatively moderate climate in the south (warmer than the northern area of Napa Valley in the winter and cooler in the summer) and a relatively extreme climate in the north (hotter than the southern area of Napa Valley in the summer and colder in the winter).
Soils The "General Soil Map" of Napa County, California, prepared by the
United States Department of Agriculture (U.S.D.A.).
Soil Conservation Service, shows most of the Napa Valley floor as being generally the same types of soils. These soils are the Bale-Cole-Yalo series which are nearly level to gently sloping, well drained and somewhat poorly drained loams, silt loams, and clay loams on
flood plains,
alluvial fans, and
terraces. In addition to the Bale series, the Pleasanton soil series dominates much of the central section of the Napa Valley floor. Both of these soil series consist of deep, alluvial soils. According to Associate Professor Deborah L. Elliott-Fisk. Department of Geography,
University of California, Davis, the high frequency of
clasts from
Sonoma Volcanics in the Oakville fan soils unifies the Oakville viticultural area and distinguishes it from Rutherford. The contribution of small percentages of
metamorphic clasts (such as
serpentine and
chert) on the Rutherford fan soils contributes to minor soil differences between the Rutherford viticultural area and Oakville. The composition of these types of minerals and rocks tends to raise the
soil pH slightly in the Rutherford area and alters soil texture and
plant nutrition. After a review of the entire record in this matter, including all data submitted pursuant to the public hearing, ATF believes that there is sufficient evidence with respect to name, boundaries, and geographical features to warrant the establishment of the Oakville viticultural area. The occurrence of specific soil types can be related to topography in Napa Valley, as topography is one of the five variables that controls soil formation. The Soil Survey of Napa County, California [hereinafter Soil Survey], published by the U.S. Department of Agriculture Soil Conservation Service in 1978, divides the 11 soil associations of Napa County into two general categories: lowland depositional soils, which account for four of the 11 soil associations and are found on alluvial fans, floodplains,
valleys and terraces; and upland residual soils, which account for the remaining seven soil associations, and are found on bedrock and colluvially-mantled slopes. The "General Soil Map" from the Soil Survey shows the location of these upland and lowland soils. This map as well as the text of the Soil Survey show that the lowland-upland soil break occurs at around the elevation. This same elevation line has been used, with one exception, to differentiate the Oakville viticultural area from the mountains to the east and west. As one proceeds down Napa Valley, Zinfandel Lane marks the widening of the valley floor, which continues until the appearance of the Yountville Hills at the southern end of Oakville. Part of the southern boundary of the Oakville viticultural area is a depositional ridge which projects perpendicularly across the valley towards the Yountville Hills. This ridge is located at the narrowest point between the Yountville Hills and the Mayacamas Range. To the north of this ridge, streams drain towards the northeast, and to the south of this ridge streams drain to the southeast. The ridge, which is at an overall elevation of around , thus functions as a drainage divide. ==Wineries==