Benthic vertical hydraulic gradients in Upper Klamath Lake, Oregon, 2017

Groundwater piezometers and lake stilling wells were deployed as paired sets at 10 locations in Upper Klamath Lake in south-central Oregon from May to October 2017 to measure hydraulic heads in and beneath the lake. Continuous water-level data from piezometers and stilling wells were then used to calculate the vertical hydraulic gradient (VHG) across the sediment-water interface to determine the direction and relative magnitude of the movement of water between the lake and underlying sediments. Over the study period, heads in lake-bed sediments closely tracked lake levels, both decreasing from spring into autumn. Instantaneous VHG was highly dynamic at all sites and exhibited high-frequency (less than 1 day to less than 1 hour) variations in magnitude and direction. Instantaneous and weekly mean VHG values often exceeded, but were commonly within, the range of measurement uncertainty (VHG less than +0.009 foot per foot [ft/ft] and greater than -0.009 ft/ft). 63 percent of instantaneous VHG values and 66 percent of weekly mean VHG values were within this range. Study period mean VHG was within measurement uncertainty at seven of the nine sites that had continuous water-level data, but two littoral sites (LC03 and LS01) had positive (upward) values greater than measurement uncertainty and are likely locations of vertical groundwater seepage. Data collected in this study provide new information about the hydraulic conditions at the sediment-water interface in UKL and demonstrate that sediment-groundwater exchange in UKL is spatially and temporally heterogeneous.