Tuesday, May 7, 2024

Matilija canyon sedimentation

This winter’s rains deposited more sediment into the former reservoir created by Matilija dam. A large 36” drain pipe has been rehabilitated to keep the water level seven feet below the crest in compliance with dam safety requirements. 

Matilija reservoir  4-01-2024

Matilija reservoir 4-25-2024

The photos above show the effects of the limited drawdown.  By April 25, Matilija Creek had reestablished a channel through the reservoir sediment.  Although the water entering the reservoir was visibly clear, some erosion near the outlet was still affecting water quality downstream.  

drone image showing the outlet of Matilija Dam, 4-25-2024
(Smitty West on Facebook)

The photo below shows the confluence of clear water flowing in from North Fork Matilija Creek on the left and Matilija Creek downstream of the dam entering from the right.  These impacts are less than when Matilija Reservoir Drained in 2020 because of the earlier season and higher flows.   

Matilija-NF Confluence 4-25-2024

The upper reservoir has experienced significant aggradation in the past two wet years.  Large quantities of sediment initially mobilized by the Thomas Fire has arrived at the relatively flat plain upstream of the dam. The extensive riparian forest growing on reservoir sediments has trapped the coarse sediment, cobbles and boulders as well as logs and debris, raising elevations above the level of Matilija Canyon road.  County road crews built an earthen berm to try to keep Matilija Creek out of the road, but the low spot floods with rainfall.  Local residents have created a temporary detour for lower clearance vehicles while trucks just plow through the puddle.  Matilija Canyon road remains closed to non-residents limiting public access to the National Forest at the head of the canyon.

Matilija Road upstream of the dam 4-1-2024

Matilija Road upstream of the dam 4-1-2024

Matilija Road upstream of the dam 4-1-2024

Further upstream, Matilija Creek has meandered and undercut the road.   

Damage along Matilija Canyon Rd, 4-1-2024

Damage along Matilija Canyon Rd, 4-1-2024

These locations are shown on the aerial image below.  It appears that Matilija Dam is having an impact upstream along Matilija Creek as the riverbed slope adjusts to accumulating sediment. 

Overview of Matilija Creek above Matilija Dam
(Google Maps May 7, 2024)

Also of interest is the large alluvial fan deposit at the mouth of Rattlesnake Canyon.  This also buried the road, but tributary inputs like this will ultimately shape the alignment of Matilija Creek once the dam is removed.  Of course, leaving the dam in place will further exacerbate all of these upstream problems.

Overview of Matilija Creek/Rattlesnake Canyon confluence area
 of Matilija Reservoir (now forested) above Matilija Dam
(Google Maps May 7, 2024)