American Viticultural Areas (AVAs) are wine grape-growing regions designated by the U.S. Alcohol and Tobacco Tax and Trade Bureau (TTB). According to the TTB, the boundaries of an AVA should enclose a region that has features related to climate, geology, soils, and physical geography that affect viticulture in ways that are distinctive relative to areas outside of the boundaries. Based on these criteria, it could be assumed that an AVA would be fundamentally based on a unique terroir. However, most AVAs are relatively large areas that encompass significant variations in soils, geology, and climate. The TTB has also permitted AVAs to be bounded by highways, utility lines, irrigation ditches and other cultural features that in most cases do not correspond with the limits of the physical characteristics (e.g. soil series boundaries, geologic contacts) that define a terroir. The creation of boundaries that correspond precisely to the limits of essential terroir-defining characteristics is actually impeded by TTB requirements, which restrict petitioners to lines derived from features that appear on USGS topographic maps. Sinuous contacts such as those that separate adjacent soil series or geologic units can only function as AVA boundaries if approximated by straight-line segments. Therefore, because of their size and crudely defined boundaries, AVAs are not based on terroir, but instead represent collections of terroirs assembled for the purposes of marketing the wines of a particular region.

An opportunity to counter this model was afforded by the owners of vineyards located in the alluvial fan of the Walla Walla River, an area that has produced some of the most critically acclaimed wines in North America. A petition to the TTB drafted at the request of these viticulturalists utilized topographic, geologic, and soil maps to define an AVA that is remarkably homogenous with respect to geomorphology, geology, and soils. Encompassing 1524 hectares (3767 acres), the area within the boundaries of the "Rocks District of Milton-Freewater" AVA lies entirely on one landform (the Walla Walla River alluvial fan), has Quaternary alluvium as it's only geologic unit, and a single soil series (the Freewater) comprises 96% of its soils.