Tidal Inundation


After the restoration construction was completed, USGS installed continuous data loggers through the Nisqually Estuary to measure water level, temperature, and salinity. These loggers were placed in historic channels within the restoration site and at three major waterways (Nisqually River, McAllister Creek and Red Salmon Slough) surrounding the restoration. Two loggers were installed on each waterway: one in the upper reaches of the river or slough and the other near the mouth where it empties into Puget Sound. Having data loggers in both locations will enable scientists to compare the rise and fall of tides simultaneously at the mouth and upper reaches. The hydrographs created with these data illustrate lag times between locations (if present) and at what tidal predictions the entire slough is completely inundated and evacuated. Tidal hydrodynamics are compared inside and outside of the restoration and also between predicted tides at NOAA’s Dupont Wharf, Nisqually Reach tidal gauge station.

ChannelRestoration_YilMeHu_1878ANDPost2006

This image shows the historic tidal channels (in blue) overlaid with restored tidal channels (in red) resulting from dike removal. Historic conditions are representative of conditions in the late 1800s, prior to the construction of the Brown Farm Dike. Source: Nisqually Indian Tribe

The hydrograph below details early changes to Leschi Slough after the reintroduction of tidal flow September 2009. Prior to dike removal, tidal influence was non-existent as indicated by the flat blue line on the graph. The first Puget Sound tides entered the historic channel on September 28th, 2009 at 3:40 pm during an incoming tide. The tidal range increased over the first week, but as of mid-November, still did not match the predicted tidal range for the region (green line). The average daily tidal range (daily max – daily minimum tide level) for Leschi Slough was limited to 4.3 feet between September 28th and November 19th, whereas, the Dupont Wharf predicted average daily tidal range was 13.6 feet during the same time period.

Figure 2. Comparison of tidal range between newly restored Leschi Slough and NOAA’s Dupont Wharf, Nisqually Reach tidal gauge station.

Figure 2. Comparison of tidal range between newly restored Leschi Slough and NOAA’s Dupont Wharf, Nisqually Reach tidal gauge station.

Along with freshwater inputs, the rise and fall of tides are the medium of energy exchange in estuary systems. They carry nutrients and sediment, create elevational and salinity gradients, and provide access to the marsh for fish and other aquatic organisms. Understanding tidal inundation in the restored area helps scientists understand the nutrient loads delivered, as well as sediment accretion rates, to the restoration sites.