Sediment Movement and Geomorphic Change Monitoring

Duckabush River near the mouth looking downstream at a location just upstream of the highway bridge. (R. Murphy/PNPTC)

Concern is growing over the potential negative effects that a changing climate might have on critical estuarine and riverine habitats. There is uncertainty over how potential sediment transport rates and geomorphological changes might impact shellfish beds, salmon habitat and water quality in these systems, of which Tribes are deeply concerned. Unfortunately, there exist large gaps in the understanding of sediment movement, mapping, and measurement in our local watersheds, so the trends and direct effects are largely unknown, especially in the nearshore areas. The Point No Point Treaty Council is working on a multi-part project to employ Structure from Motion (SfM) and suspended sediment concentration (SSC) analysis in several Hood Canal rivers and associated estuaries in order to assess geomorphological and sedimentation changes through time. The overarching goal of this project is to be able to better inform aquatic habitat protection and management decisions through sound scientific research.

Structure from Motion (SfM)

Sediment and structural movement in streams and in estuaries is of vital importance to the health of aquatic habitat. Productive shellfish beds need a sediment supply to survive, but too much sedimentation or too much movement of estuarine and nearshore geomorphological structures and substrate can be detrimental to shellfish productivity and survival. Sediment changes can also impact finfish health, particularly in the juvenile stages in the estuarine environment. In order to assess areas of vulnerability, it is necessary to gain an understanding of current and historical conditions through detailed mapping and data collection. PNPTC staff are applying SfM protocols on the Duckabush, Dosewallips, and Hamma Hamma estuaries and their associated freshwater river environments to examine morphological changes through time.

Conceptual cartoon of the Structure from Motion methodology. Numerous pictures of the geomorphic structure are taken using a handheld camera from various angles and elevations. A 3D model or DEM can later be constructed from the images.

Suspended Sediment Collection

While the SfM methodology outlined above is a great way to assess general geomorphological changes through time, estimates of suspended sediment rates and sediment budgets in many of the Hood Canal river systems are not readily available. Measuring SSC can be an important indicator for aquatic watershed health.  Higher concentrations of sediment can greatly affect the water quality, resulting in an increased turbidity, decreased photosynthesis (due to less light transmittance), egg burial, and clogged fish gills, potentially detrimentally affecting existing fish populations. In 2019, PNPTC staff began a water quality project to assess SSC at two locations along the Duckabush River and are expanding the sampling program to include locations in other watersheds as well.