May 27, 2019

The Fraser River Plume

One of the most prominent features in satellite pictures over the Pacific Northwest is not meteorological at all, but associated with a river:  the Fraser River plume.

Each year, snowmelt and precipitation pick on materials in the mountains, which  then flows down the Fraser River Valley towards the Strait of Georgia and the Pacific.  The silt-laden river takes on a milky or even brownish color, which is very evident in satellite pictures (see  MODIS satellite imagery from a few days ago, below).

The plume boundaries can be very sharp, as illustrated by this picture taken by Aaron Donohue from the deck of the Nanaimo Ferry.

The brackish water of the Fraser plume is less dense than the saltier water of the Strait of Georgia and thus floaters on top in a thin upper layer.

The Fraser River, by the way, is the longest river in British Columbia, stretching nearly 900 miles towards the northeast, and draining a huge area (see map).  Much of the region includes lots of unconsolidated debris left over from the last ice age.  Add that to the heavy precipitation of British Columbia, leads to  a potent combination for moving substantial amounts of silt and material into the Fraser and then into our coastal waters.

Importantly, the silt coming out of the the Fraser plume plays a critical ecological role, essentially fertilizing the coastal waters of BC, northwest Washington, and the coastal Pacific.  Specifically, the Fraser outflow provides nutrients (such as silica and iron) that aids growth of plankton, which in turn supports the great productivity and biodiversity in the Strait of Georgia and nearby waters.   The health of the salmon fishery in the region is dependent on Fraser sediment.

Finally, we should not forgot the substantial impact the Fraser River Valley has on our local meteorology during winter, providing a conduit of very cold air into western Washington during cold air outbreaks.   Anyone living from Bellingham northward knows about the Fraser outflow winds.


  1. Those of us living on the NE portion of the Olympic Peninsula, particularly near Sequim, know the Fraser River Outflows very well as well.

    Most of our wintertime snowfall occurs during Fraser Outflow conditions when our side of the Olympics switches from being downslope to upslope. Also our coldest temperatures. Outflow comes right across the Strait to us.

    As in, 30" of the heavy stuff this last February on top of Bell Hill, which stayed for weeks.

    It's the "reverse rain shadow" effect. We watch the weather in the Fraser River Valley very closely in winter.

  2. How interesting, Cliff!

    I never knew about the importance of the Fraser River's silt. I'm very pro salmon and all that affects the species' success.

    Thanks for pointing this out in your blog. Hope you have had a good holiday weekend.

  3. The waters were quite silty after leaving active pass on the ferry yesterday from Swartz Bay to Tsawassen. There were whales feeding in the distance as well.

  4. I have some friends that have a second home in Boston Bar, BC (in the Fraser Canyon), so I'm quite familiar with that river.

    There are no dams on the mainstem of the Fraser, which helps it to be siltier (reservoirs act as giant settling ponds). It's amazing how much it can rise during the spring floods, even in very swift-flowing regions, such as the Fraser Canyon. 40 or more feet of rise, even in sections that border on being rapids! It's a simply staggering volume of water per second. Visiting in the fall, it's hard to believe how much higher the river was in the spring.

    Odds strongly disfavor this year's Fraser spring flood being stronger than last year's, which was a record-breaker. The river got up into forested areas that typically don't flood, and no small amount of trees perished because of it.

  5. Hi Cliff, I've been following your blog for years and I always appreciate your knowledge and perspective. I am curious though what you think about the density of the silt in the sea in relationship to agressive clear cutting forests, especially in the BC interior.

    I have spoken with a number of Lummi folks who claim that the nooksack Delta, for instance, has experienced massive clogging from silt ever since clear cut logging practices were introduced to the area. Add to that the fact that Canada permits logging much higher into the sub-alpine than we do in the states, and I would think that this could have a significant impact on the spring silt plume, even though it is a perfectly natural phenomenon to begin with.

  6. Do you the the dams on the columbia river drainage collecting silt in the reservoirs instead of bringing it to the ocean is a major factor in the decline of the salmon runs in that system?

  7. Travis - Another factor is dams/silt and jetties/silt at the mouth of the Columbia. Since the mid 1940s the shoreline has advanced a couple thousand feet on the southern part of Long Beach Penninsula, and retreated to the north. Anyone have a link to studies on this?

  8. The Puyallup River dumps a lot of glacial sediment into Puget Sound at Commencement Bay. Elliott Bay used to get some of that glacial till until the White River was diverted from the Duwamish system to the Puyallup around 1910.

  9. Speaking of river plumes, and salmon since you mentioned it, Vancouver island rivers are so low that at least one emergency has been called - an evacuation of salmon fry from stranded pools that are heating up under the same relentless dryness that has been decimating the salal bushes since January.

    They are helicoptering salmon fry from the withering rivers....... in May!

    all under a now rather normal haze of wildfire smoke...... but this time, in May!

    I'd say our salmon are going to need a little bit more than the usual silt from the Fraser to survive this era.

  10. Fraser outflow also controls circulation of oceanic water in the Strait, and into Puget Sound, which is another ecological variable that contributes to bottom up marine conditions coincident with juvenile salmon outmigration that determine early marine survival -- one of the most important periods that determine the likelihood of surviving to adulthood.

    The Si:N ratio determines the structure of the marine food web that cascades up to salmon. We still have a lot to learn about this considering how many interacting variables are at play, but nitrogen inputs from wastewater treatment plants may be driving the marine food web toward phytoplankton assemblages that are poor at transfering energy up the food chain. One more reason to increase funding for scientific research!

    If you want to help, call your electeds, and tell them how important it is to increase funding to understand whats happening to marine ecosystems, salmon, and orcas.


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