Wednesday, February 3, 2016

A real flood season at Elwha

The plot above of river flow from the McDonald Bridge gage on the Elwha River looks so very different then that from previous winters. We've had multiple flows above 10K cfs, three that have exceeded 20K, and one that went above 30K cfs. The river has spent a good bit of time above the median flow based on records dating back >100 years.

I've got a camera up at Fox Point watching the river mouth, and its been a fun ride this winter will all of this water pushing through. The mouth has been making a lot of changes...check it out below. What you see here are the average of individual photos taken every 30 minutes during daylight hours. This helps to adjust for variations in lighting and water level associated with tides, and better visualize the actual morphology change at the river mouth:

The timeline of this video aligns with the plot above - 90 days. I've also added notations to the slides on days when flow falls into one of three bins: >10K cfs (but less than 20K), >20K cfs (but less than 30K), and >30K cfs.

In particular what I think is cool is that the video suggests that the river is building new bars (visible by the end of the video emerging above the waterline) at locations seaward of the mouth position in early November...probably due to the at the mouth building on top of sediment that was pushed off of the delta during the large floods of mid-November. To better control for water level here is a photo taken at the the lowest part of the daylight tide on 6 November 2015..the water level was about 0.8 m MLLW according to the P.A. gauge:

and here is a photo taken at the same water level (+/- 10cm according to the P.A. gauge) on 1 February 2016:

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