The distribution of water in the upper mantle is believed to have strong influence upon global dynamics by influencing mantle rheology, modal mineralogy, melting systematics and chemical differentiation. The principal input of water is the process of subduction which is estimated to have introduced several ocean volumes to the mantle over Earth history. In principal, a large proportion of this water may be dissolved in the nominally anhydrous silicate minerals (NAMs), but quantifying this has been challenging. Xenoliths only sample the upper 200 km, less mostly, and suffer concern about alteration on the way to emplacement. Recent laboratory data show that seismic velocity is not sensitive to intracrystalline hydration. However, electrical conductivity is strongly sensitive and could provide estimates of mineral water content with suitable constraints. An 1300 km E-W transect of ~400 magnetotelluric (MT) soundings spanning the period range 0.01 to 17,480 s has been acquired from the northern California coast over the Gorda plate, across the Great Basin of Nevada and western Utah, and spanning most of the Colorado Plateau of eastern Utah. Regularized 2D inversion reveals an upper mantle whose resistivity below the broad Great Basin falls progressively with depth from values of ~100 ohm-m near 50 km to <10 ohm-m by 400 km depth. We test the hypothesis that the vertical resistivity profile is consistent with the maximal hydration degree allowed by ambient T-P short of triggering H2O-undersaturated melting (cf. Ardia, 2012, EPSL). An obvious possible source of hydration would be the Gorda, and to some extent the prior Farallon, subducting plates. Assuming standard and enhanced adiabats, deep resistivity profiles predicted using lab data of Novella (2017, Sci Rpts) suggest only resistivities in the near ‘hanging wall’ of the Gorda subduction zone under northwestern Nevada are low enough to represent full NAMs hydration. Under the central (eastern Nevada) and eastern (western Utah) Great Basin, large-scale resistivities are 2-3x too high, nominally. However, channelization of fluid upward from the plate could mean a mixed saturated-unsaturated peridotite upper mantle. Support has been from U.S. Dept of Energy contract DE-0006732 and National Science Foundation grants EAR-0838043 and OPP-1443532, and numerous prior.