Thursday, August 13, 2020

Retrospective on the Thomas Fire

The Ventura River has demonstrated amazing resiliency following the Thomas Fire.  The river appears healthier now than it has been in decades following the influx of sediment eroded from the mountains.  However, the initial flushes following the fire negatively impacted the river until the storms of 2019 "rolled" the bottom breaking up an impervious layer formed by ash and silt deposits.  The photos here are provided to illustrate what this looked like in the summer of 2018.

Ventura River, May 2020

Ventura River following the Thomas Fire, January 2018

Ventura River following the Thomas Fire, March 2018

The first storm immediately following the fire in January 2018 moved large amounts of black ash and very fine organic matter off the burnt hillsides. The second storm in March eroded additional fine sediments with a brown soil color.  These two events combined to form a hard crust on the stream bed.

The photos below from June 1, 2018 describe observations of this phenomenon. 

Ventura River 6-1-2018

The river bottom was embedded with ash and fine sediment that filled the voids and effectively "cemented" the cobble and gravel.  In the image above, a cobble was removed by hand to expose the layers below.  With no tools handy, it took considerable effort to loosen and remove this rock.

Ventura River 6-1-2018

Once a rock was removed, the hard crust could be broken up and pulled away by hand, further exposing the riverbed.  Here the substrate was agitated to allow the river flow to transport the fine sediments away.  The dark color of this plume indicates the organic ash eroded from the hillsides in the initial rain event.

Ventura River 6-1-2018

Removing a couple more rocks and agitating the surface eventually cleared this small area of river bottom.  The brown crust can be seen adhered to the rocks.  The image below provides an overview of the site.

Ventura River 6-1-2018

The upper Ventura River groundwater basin relies on flows infiltrating through the alluvium in the river reach downstream of the Robles Diversion.  The graphs below are from the monthly water update provided to customers of the Ventura River Water District.  This water district relies on wells in the Ventura River in the general vicinity of the photos above.  As noted at the time, groundwater levels did not  recover as normally expected in 2018.  However, the 2019 storms resulted in river flows that adequately cleaned out the river channel to allow recovered infiltration as illustrated by the 44 foot rise in groundwater in the bottom chart.  

Ventura River Water District, July 2018

Ventura River Water District, June 2019

On this blog:

March Rains - fires, flood, drought, and sand