Monday, November 23, 2015

Alluvial Fans and a Disappearing Creeks

Last week I headed out to check a few creeks that I have and am doing work on. A decade ago I assessed and mapped debris flow hazards associated with these creek. Debris flows are a major hazard on alluvial fans. A landslide or blockage in the creek can send a fluid mass of debris (and in western Washington lots of logs) down onto the upper part of the alluvial fan.

The hazards associated with alluvial fans are not limited to debris flows. Post debris flow streams on the fan can be very erosive. If the debris flow reroutes the stream, rapid down cutting may take place depending on the gradient of the new stream route. Erosion from the down cutting can then be deposited further down the fan causing flooding or stream movement at a new location. 

The purpose of my venture in the heavy rain event was to see how the streams on the fans in question were behaving during a large storm event as two of the creeks were flowing on new routes and one was flowing across an area where lots of previous deposition had taken place from storm events 5 and 7 years ago.

What follows is observations on one of the creeks. 

Old stream channel

The old channel no longer has any water flowing through it as the creek has been redirected. The channel shown above is one of several old abandoned channels on the fan surface. Water was flowing through this channel as recently as 2009. I had first observed this channel in 2004 when the stream was flowing here. From 2004 to 2009 the stream down cut 5 to 10 feet deep into the fan surface at this channel site.

The existing creek now follows a new channel route that angles across the alluvial fan towards another alluvial fan to the south. Above the new channel the creek can be seen to have dun cut a narrow channel that has a bottom lined with larger rocks such that the down cutting has been reduced.

Down stream the stream flows across a fairly gentle gradient such that despite the high flow the day of my site visit there did not appear to be any erosion as the flow velocity was not high.

Grasses and the soil under the grass was still in place at this location.

A bit further downstream the stream disappears. At this point the stream was flowing onto another alluvial fan with gravels from a stream located further south. Both fans have been deposited over a gravel filled ice age river valley that readily infiltrates lots of water.

Further downstream and looking upstream to the log shown above there is no water flow - at least on this wet day with local rivers at flood stage.

A few feet from where the picture above was taken is the channel and stream of the next creek to the south.

I followed this creek as well and its flow ended rapidly much like the first as it flowed onto the gravel and cobble river bars of the former ice age river plain.

Diminished stream as water rapidly infiltrates

Last bits of the stream soak into a meadow underlain by river deposits from an ice age river

Neither of these creeks are connected to another surface stream even during flood events and as such are not fish bearing and not potentially fish bearing.

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