Flooding rivers could carry enough sediment to kill salmon.
That is why the Quileute Indian Tribe is using a 7-foot-tall crane to suspend a 30-pound submarine-shaped weight from a bridge into the raging torrent below.
The amount of sediment flowing down the Calawah and Bogachiel rivers – both tributaries to the Quillayute – has a huge bearing on salmon survival. Too much fine sediment can smother fish redds, damage fish gills, degrade habitat, and starve fish of oxygen.
The hydrodynamic design of the sampler keeps it vertical in the river while a glass bottle inside fills with water.
“We have to wait for high water, which means we’re normally working in a rainstorm or just after a storm has passed through,” said Nicole Rasmussen, habitat biologist for the Quileute Tribe.
The study team operates the crane by hand and takes ten measurements at equal widths across the channel to collect a complete sample.
“We also have to be very careful using the heavy crane and weight,” she said. “If we bang against the riverbed, we’ll kick up sediment and our sample won’t be accurate.”
It is also essential for the tribal scientists to collect a consistent amount of water throughout the water column. “We use a metronome so when we hand crank the weight up and down in the water, we’re going at a steady pace,” Rasmussen said.
Suspended-sediment concentrations were last measured in 1978 when the amount that flowed down just the Bogachiel River in a year weighed as much as the Empire State Building.
“A lot of the land upstream of where we’re taking samples is active forestland, including clearcutting along almost 7 miles of the upper Bogachiel,” Rasmussen said.
The investigation will eventually lead to the development of a model that should predict the sediment load in the two rivers.
“With the model, we can quickly look at the flow and turbidity at any point and figure out what is going on with the suspended sediments, and calculate annual sediment load,” Rasmussen said. Turbidity is a measure of how light passes through water but doesn’t necessarily tell the tribe what kind or quantity of salmon-killing sediment is in the river.
“You can have the same level of turbidity in the river, but the level of suspended sediments could be different,” Rasmussen said. “When you’re talking about fish, it is the fine sediments that matter.”
Natural resources staff from the Quileute Indian Tribe recover a hydrodynamic sampler from the Bogachiel River.