Water levels in the Skagit and Nisqually River estuaries, Washington

We obtained time series of water levels in tidal channels in the Skagit River Estuary (48.32029°N, 122.39529°W) from October 2017 to October 2018 and in the Nisqually River Estuary (47.08141°N, 122.70514°W) from June to December 2023 to determine local tidal datums for nearby tidal wetland research sites. We deployed Hobo pressure sensors in PVC wells and surveyed their elevations using real-time kinematic GPS in the North American Vertical Datum of 1988 (NAVD88). At the Skagit station, the sensor was at 0.130 m NAVD88 and water level was collected every 12 minutes. At the Nisqually station, sensor 20144078 was at -1.111 m NAVD88 and sensor 20183671 was at -0.930 m NAVD88; water level was collected every 6 minutes. Using NOAA CO-OPS’s on-line “Tidal Analysis Datum Calculator” with the direct method and Seattle tide gauge (Station 9447130) as the control station, we used 12 months of data to determine tidal datums (in meters relative to NAVD88) at the Skagit site as follows: mean high water = 2.593 and mean higher high water = 2.863. We used the calculator with the standard range ratio method and the Tacoma tide gauge (station 9446484) as the control station at Nisqually to determine tidal datums (in meters relative to NAVD88) with the five full months of data (July-November) as follows: mean lower low water = -0.399, mean low water = 0.189, mean tide level = 1.500, mean high water = 2.810, and mean higher high water = 3.103. Each time series also provides water temperatures.