The greatest subtlety of wetlands lies in their hydrology. This is unfortunate, given that the very name wetland invokes images of water on the land. In fact, those who have been impatient with the thicket of indicators by which federal agencies identify wetlands have at times demanded a return to basics in the form of simple inspection for water. One Washington insider even suggested that we might send out her Uncle Dennis to delineate wetlands. If Uncle Dennis, who has a propensity to sit down, returns with a wet bottom, he has been in a wetland; otherwise he has not. The truth is that Uncle Dennis could sometimes wet his shorts in an upland forest or grassland, yet at other times sit comfortably dry on a certified wetland. The hydrologic conditions that separate wetlands from uplands in fact involve four phenomena related to saturation: proximity to the surface, time of year, duration, and frequency. Proximity of saturation to the soil surface is one of the most widely misunderstood aspects of wetlands. In many wetlands, water stands on the surface for extended intervals or, in a few cases, continuously. The significance of standing water (inundation), however, is easily overrated. Inundation is neither necessary nor sufficient for a wetland. Recurrent and prolonged saturation of soil near the surface is sufficient to create and maintain a wetland, even if water never actually stands on the surface. The exact distance within which saturation must approach the surface varies from one region or one wetland type to another but by rule of thumb is about one foot or, scientifically speaking, 30 cm. While plant roots often extend beyond one foot, prolonged saturation of the upper foot blocks penetration of oxygen into the soil, and thus to roots, so completely that most plants cannot survive. Plants that tolerate or thrive in saturated soils are called hydrophytes; they are characteristic of wetlands. Thus, the critical depth for saturation is determined by the differential responses of hydrophytes and non-hydrophytes to prolonged immersion of roots in water. The upper one foot of soil is also the zone of most intense microbial metabolism in soil.