Robles Diversion and Matilija Dam

Robles Diversion has been an essential component of the Matilija Dam Ecosystem Restoration Project (MDERP) since planning began in 2000.  

Background:

The Robles Diversion and Fish Passage Facility is located on the Ventura River 2.3 miles downstream of the Matilija Dam. The Facility includes a timber diversion dam and control gates to divert flows from the upper Ventura River for storage in Lake Casitas.  The lake serves as a primary and backup water supply for agricultural and municipal water users served by the Casitas Municipal Water District (CMWD).  

The diversion was originally constructed by the U.S. Bureau of Reclamation in 1956 as part of the Ventura River Project which is built around Casitas Dam and Reservoir, located on Coyote Creek about two miles above its junction with the Ventura River.  Casitas infrastructure also includes the 5.4-mile Robles-Casitas Canal, which conveys the diverted flow of the Ventura River into Lake Casitas; and the main conveyance system, which includes 34 miles of pipeline, five pumping stations, and six balancing reservoirs located throughout the project area.  When originally constructed Lake Casitas had a capacity of 254,000 acre-feet, but recent estimates have reduced this to approximately 237,000 acre-feet. 

Robles Diversion and canal
Jan 2003, Rich Reid

According to CMWD, Lake Casitas fills primarily from the natural flow of local creeks and the Robles Diversion. In any given year roughly 1/3 of the Lake’s water comes from the Robles Diversion. 

Diversions are limited by the size of the canal, which can only convey 500 cubic feet per second (cfs) and regulations in a NOAA biological opinion that require adequate releases of water to ensure fish passage and other downstream water supplies. 

The Coyote Creek watershed area that drains directly into the lake is only 3,500 acres. (In comparison, the similar sized Lake Cachuma (193,304 acre-feet) in the neighboring Santa Ynez watershed is supplied by more than 70 times the watershed area of 417 square miles (266,880 acres).)   It was this small watershed area that necessitated the addition of diversion from the Ventura River to help maintain the water supply in Lake Casitas. 

In 2005, a fish passage facility was constructed to allow migration upstream and downstream as well as to screen smolt and other small fish from being diverted into the lake. The components of this facility are shown below:

Robles Diversion components
source: Casitas Municipal Water District

Sedimentation Problems:

Robles Diversion - the flood filled the forebay with sediment and breached the timber cutoff wall
Casitas Municipal Water District quickly implemented emergency repairs to restore water
 diversions to Lake Casitas

As the Robles Diversion dam captures water for diversion in the large forebay, it also traps sediment.  This sedimentation requires ongoing maintenance, including the costly mechanical removal and transport of sediment following a series of small storms or, in some cases, one big winter.

In January 2023 the upper Ventura River experienced a 50-year flood with flows over 20,000 cfs in the Robles reach.  This occurred following the Thomas Fire of December 2017 which burned the majority of the upper watershed.  A huge volume of sediment, including cobble and boulders, backed up behind the diversion dam filling the forebay.  Flows became diverted to the east, eroding the earthen berm up until, fortunately, the timber wall failed.   A similar event occurred in 1969, when 100-year flood flows were diverted both east and west (a phenomenon known as "avulsion") damaging both the diversion canal and the adjacent "riverbottom" neighborhood of Meiners Oaks.  Diversion capability was not restored for the remainder of 1969.

As described in Matilija Dam: removal plans and sediment transport, this existing sedimentation problem will become worse as sand, gravel, and cobble begin to pass over the dam.  Even if Matilija Dam remained in place, future equilibrium sediment loads will triple from what Robles has experienced over the past 70 years.  And dam removal will create a short term sediment "wave" that could also overwhelm the facility.

Solutions to increased sedimentation:

The 2004 Army Corps of Engineers feasibility study for dam removal included a "High Flow Sediment Bypass" at Robles Diversion.  This bypass consisted of additional gates that could be opened to allow sediment transported during peak flows to be flushed through this reach of the river, preventing the kind of backup that occurred in 2023.  

 

In 2019, the Robles Working Group was formed to aid planning and implementation of modifications to the Robles Diversion Facility (operational and/or physical) as an essential component of the Matilija Dam Ecosystem Restoration Project (MDERP).  A consulting team is facilitating and providing technical support for the RWG. The RWG is currently focused on evaluating and selecting an alternative for the Robles Diversion Facility that will enable it to function reliably for water supply, sediment passage, and fish passage.

This work has determined that 

• Existing bypass is too narrow, less than 30% of natural river channel

• Matching natural river width at Robles would require additional bypass between 130 ft to 160 ft wide. 

• It’s feasible to increase flow and sediment transport capacity at Robles

• Possible to adjust flow split between existing bypass and additional bypass as desired

by modifying invert elevation and forebay layout

• Various options to locate additional bypass and change approach flow conditions by

modifying forebay (narrowing, straightening, splitting)

Several alternatives were considered and narrowed down based on additional computer analysis.  Currently the Bureau of Reclamation has built a physical scale model in their facility in Denver to test various configurations and determine the best approach for optimum sediment management.

The two primary alternatives #3 and #5 are illustrated below:

Fisheries considerations:

Although the initial intent of the Robles Diversion modification effort was to address the sedimentation concerns experienced during high flows, the resource agencies have a strong interest in improving fish passage around the diversion facility.  Currently the fish ladder is the only opportunity for passage, even during times when the diversion is not operating.  All river flows are currently routed through the diversion facility down the fish ladder with no bypass option, even outside the diversion season.  And although the fish ladder has proven to be effective for trout, there is interest in ensuring other native species can pass.  Additionally, the current operations are governed by NOAA's biological opinion, which defines how much water is allocated to the fish passage operations.  All of this adds considerable complexity to an already difficult engineering task.   Considerable resources are being directed to the exploration of additional fish ramps or other passage facilities to resolve these concerns.

Flooding concerns:

The original Corps of Engineers plan for this reach included construction of a new levee extending downstream from the Robles Diversion.  This was based on coarse 1-dimensional flood modeling that predicted an incremental increase in streambed elevation due to dam removal.  

Apart from the huge construction footprint, local residents did not welcome the prospect of a large wall complete with chain link fences, rodent control, and service road in their back yards.  It turns out much of the flood concern was based on unrealistic backflow from the Ventura River.

Subsequent 2D modeling provided a more realistic understanding of the flood concerns at Meiners Oaks.  As described above, sediment trapped by the Robles diversion dam creates a flood hazard to the adjacent community when the river is diverted by the blockage (avulsion.)  The consulting team has conducted a preliminary study of how these high flow bypass designs will affect the downstream flood risk.  Although the existing flood risk from the Ventura River may be characterized as relatively minor overtopping of the naturalized embankment, initial assessment shows that enhancing the flow path through this reach will help reduce this potential flooding.  (Note that a considerable area of this neighborhood is at risk from overflow from Cozy Del creek, shown in yellow.)  These preliminary results are promising, although the primary flood risk is potential stream bank erosion on the left (east) bank of the existing high flow channel downstream of the Robles Diversion.  This downstream reach is directly affected by how flows are directed by the upstream modifications. 

Summary:

The Ventura River Project constructed in 1959 provided Casitas Municipal Water District more than the 50 year lifespan of benefits from Matilija Dam.  Over that period, the district was able to release water stored in Matilija reservoir for diversion at Robles into Lake Casitas.  Sedimentation has rendered Matilija Dam completely useless for water supply, and structural degradation requires its timely removal.  Most importantly, dam removal necessitates modernization of downstream infrastructure. 

Modification of the Robles Diversion facility has been identified as the critical path component of the Matilija Dam project.  Although complex and expensive, careful consideration of a solution that eliminates costly maintenance and periodic cleanout of sediment from the diversion facility as well as the need for costly maintenance of a permanent levee downstream will provide long term benefits to the community.  Completing this component will clear the path to dam removal which is necessary to restore watershed resilience including access to headwater habitat that the endangered steelhead require.  

References:

https://matilijadam.venturacounty.gov/robles-diversion/

https://www.casitaswater.org/learn-about-diversions

https://www.venturariver.org/2023/02/january-2023-flood-overview.html

Matilija Dam: removal plans and sediment transport

 

Matilija Dam, aerial view of impounded sediment, May 2024

Planning for the removal of Matilija Dam has been ongoing since 1998 (see chronology report).  As with all dam removal projects, sediment management is the primary concern.  Questions include how much sediment has accumulated, how best to remove the sediments to restore the natural stream gradient, and what are the downstream impacts when sediment is released from the former reservoir?  

This last question is particularly relevant to the Ventura River where significant floodplain and water supply infrastructure has been developed since Matilija Dam was constructed in 1947.  Concerns about potential impacts to aging and deficient levees downstream as well as the Robles diversion have led to a multicomponent project called the Matilija Dam Ecosystem Restoration Project (MDERP.)  Although complicated by the multiple agencies, interests, and components, this complex endeavor will ultimately enhance watershed resilience through upgraded infrastructure and restored natural processes.

Map of MDERP project components,
Ventura County 2024 

Field studies have confirmed the general profile of sediments trapped upstream of the dam.  Over the decades, the remnant reservoir accumulated fine silt and clay while the larger "coarse" sediment accumulated in the delta and upstream channel.  

Note that over the past 3 decades since studies began, it is estimated that approximately 50% more sediment has accumulated behind the dam, primarily due to the 2018 Thomas fire and the floods of 2022-2023.  Currently, sand and gravel has overlain the fine reservoir deposit and filled to the level of the outlet on the back side of the dam so that sand and gravel are now being transported through and over the dam.  

Current planning for dam removal includes engineering two 12ft diameter orifices that will be opened during a flood to allow high flows to head-cut a channel upstream through the sediment (AECOM, March 2022).  This initial flush will mobilize a large percentage of the fine sediments stored in the former reservoir area resulting in temporarily high turbidity (i.e. "muddy" water) downstream.  

Dam removal concept for Matilija Dam

The plans call for complete removal of trees and other vegetation upstream of the dam and excavation of a "pilot channel" to initiate head cutting through the middle of the reservoir area to maximize evacuation of the fine sediments during this initial flush.  It is expected that this channel will become relatively stable and erosion will diminish once the flow cuts down to the approximate depth and alignment of the historic channel.  A storm prediction analysis (AECOM, November 2022) determined the preceding conditions necessary for a 3-yr or greater flood event, equivalent to about 3,300 cfs deemed necessary to adequately establish the new upstream channel and flush the fine sediment down the Ventura River and out to the ocean. 

channel through sediments upstream of Matilija Dam resulting from head-cut erosion during flood
alignment based on historic streambed (Stillwater 2016)

After this initial (phase I) flush, future floods will incrementally move more of the coarse sediment (gravel, cobble, boulders) down the Ventura river.  Based on other similar dam removal projects, it is assumed that a maximum of 80% of the trapped sediment will eventually be transported downstream.  In other words high terraces will exist adjacent to the restored stream channel through Matilija canyon that will gradually erode over time feeding sand gravel and cobble into the Ventura River in addition to the restored natural "background" sediment flowing down Matilija Creek.

Estimates of sediment available for transport after dam removal

A computer model was developed to simulate the downstream response of the Ventura River to the release of sediment resulting from dam removal (Stillwater Sciences, February 2020, March 2025).  The table below shows the sediment volumes applied to this computer simulation.  Note that the impounded sediment represents approximately 7 times the "background" supply from Matilija Creek, which makes sense since the dam has been trapping that supply for decades.  Also note that the supply from North Fork Matilija Creek (which runs up Hwy 33) is just one third of the background volume from Matilija Creek.

Sediment supply applied to computer models of the Ventura River

The change in sediment load in the Ventura River may be conceptualized by the graph below.  The current trend with dam in place is for increased sediment to pass through/over Matilija Dam until the background load from Matilija Creek is added to the current sediment transport in the river.  The initial phase of Matilija Dam removal will generate a large pulse of predominantly fine sediment released through the orifices (outlets) in the base of the dam which will be transported by flood flows down the river and out to the Pacific ocean.  After this initial flush, the dam will be deconstructed and each subsequent flood will erode and transport coarser sediment from the reservoir delta and upstream channel until long-term equilibrium is reached. There may be 6 or 7 times the current sediment load during initial release of sediment, and approximately three times current load with long-term equilibrium conditions.  Restoration of natural sediment transport will provide tremendous benefits to the riverine ecosystem, migrating endangered steelhead, and ultimately the rivermouth delta and beaches of Ventura County, but some of the downstream infrastructure will need to be modified to accommodate these changes.  

In order to model sediment transport, several "synthetic hydrographs" were developed based on historic flows in the Ventura River.  Because the "flashy" watershed is characterized by long periods of dry conditions with occasional large winter floods, hydrographs were developed to compress time and represent those periods when sediment is moving down the river.

 The "Run 2b" hydrograph represents the most rapid transport of sediment from the reservoir by repeating the extreme floods of 1969 back-to-back four times.  The "Run 2e" hydrograph was used to help predict what may happen under a more realistic scenario.   The graph below illustrates the rapid flushing of sediment from Matilija Dam with the 2b hydrograph and the relative magnitude of this volume compared with the background inputs from Matilija Creek and other tributaries.

The map below gives an overview of the extent of the computer model.  Topography based upon Lidar imaging provided the base channel configuration.  Note that modeling is limited to a "fixed bed" so the natural scour and avulsion (channel changes) are not taken into account. 

The graph below shows the maximum expected downstream changes to the riverbed and 100-year flood elevation with the removal of Matilija Dam.  As one might expect, the biggest changes are in the first two miles directly downstream of the dam where the river channel is narrow and constrained.  Below the Robles diversion the floodplain widens and becomes braided with a natural capacity to absorb increased sediment loads.  Note that the model assumed that modifications are made to both the Robles Diversion and Santa Ana Bridge to accommodate the increases in sediment.  (Santa Ana Bridge widening project was completed in 2022 and planning is underway for a high flow bypass at Robles.)  

Over the course of the project, several computer modeling approaches have been used beginning with a one dimensional model, the DREAM2 dam removal model, and several iterations of the 2-D model which was developed to help provide greater resolution for areas of concern.  As illustrated below, all of these models show similar trends.    

These results may be translated into more practical terms by projecting the predicted 100-yr water surface elevation onto the floodplain topography to visualize potential changes in flooding.  Below is one example of the worst case scenario in the reach directly downstream of the dam.  The yellow area depicts the potential change in the floodplain due to dam removal. 

The next step will be translating all of this data into a risk and liability approach that considers all downstream interests.  Developing a realistic assessment of these risks will be important step toward dam removal.

Update:

The modeling report was updated in December 2025 to include a worst case probability analysis based on a model sensitivity analysis;  

Multiple SRH-2D model runs were conducted during Phase III to assess the sensitivity of downstream impacts to different rates and volumes of sediment evacuation from the Matilija Dam impoundment.  ...the sensitivity analysis for the upstream model was primarily conducted with Run 2e hydrology because it represents a much more probable hydrologic scenario as compared to Run 2b. For Run 2e, seven sensitivity runs were conducted including sediment supply volume/rate increases of 10%, 20%, 30%, 40% and 50% and decreases of 20% and 37%. For Run 2b, three sensitivity analysis runs were conducted including sediment supply volume/rate decreases of 14%, 20% and 32%. No sensitivity analyses were conducted assessing the impacts of sediment supply/rate increases for Run 2b because it is already considered to be a rare and unlikely hydrologic scenario in terms storm event magnitude and the short duration of reservoir flushing. 

A statistical evaluation was conducted to determine which of the sensitivity analysis model runs most appropriately represent the 80th-percentile (P80) behavior observed in the ensemble of simulations. 

The sensitivity analysis and subsequent statistical evaluation provide an estimate of plausible upper-bound project impacts for the Study Reach based on a composite of model results from different model simulations as described below in Table 5-1.  Run 2e + 30% represents a closer match to the P80 impacts closer to the Matilija Dam and Run 2b represents a closer match farther downstream. 

The timing of these worst case conditions are shown in the table and illustrated with arrows on the modeled hydrographs below;

The main changes to the results of the SRH-2D modeling is an increase in potential flood risk for the upstream reach of the river.

Floodplain map for upstream reach, P80 (run 2e + 30%)

Commentary: 

The Matilija Dam Removal Task 2.9 2D Modeling Report summarizes more than two decades of analysis directed at determining the transport and fate of the more than 8 million cubic yards of sediment currently sequestered in the former reservoir of Matilija Dam. This unique dam removal project in a steep flashy coastal drainage is complicated by the developed floodplain downstream of the dam. The need to assess the risk of the project to infrastructure and communities downstream has required an ongoing evolution in the state of the art hydrologic and geomorphic computer simulations.

This body of work concludes that that the primary impact of dam removal will occur in the first few miles downstream of the dam within a timeframe bounded by the first two 100 year flood events (perhaps many decades or more than a century into the future). In the upper reach of the study area below the dam the floodplain is geologically constrained and the pulse of sediment released from behind the dam will temporarily increase streambed elevations and water surface elevations. This effect diminishes downstream of the Robles/Meiners Oaks area as the broad undeveloped floodplain will naturally absorb and meter out this sediment pulse through avulsion and channel migration processes beyond the capacity of this modeling effort.

The Phase II SRH-2D model improves on the prior 1D analyses. However, the Phase III SRH-2D model that was developed to add sand transport to the simulations probably overpredicts streambed aggradation. The models developed here for Matilija Dam, particularly the Phase III analysis, represent an advancement in the state of the art, therefore there is not yet a similar case where this has been applied for comparison.  Moreover, the P80 analysis does not represent the 80th percentile likelihood of occurrence, but rather the 80th percentile of the computer model runs chosen for this analysis.

However, to the extent that other large dams have been removed, there is some hindsight into how such predictions have played out. One important conclusion from the referenced review of dam removal (Major et al 2017) is:

Rivers are resilient. After dam removals, rivers have largely achieved apparent stability – at least within their range of natural variability – within a few years rather than decades. Despite concerns about rapidly releasing possibly large volumes of sediment to river systems, most removals to date have shown adverse geomorphic impacts (such as complete bed-texture transformation, enhanced flood risk, or damage to fish habitat) are short-lived and have been neither as damaging nor as long-lasting as feared.

References:

Comments on Matilija Dam Removal Task 2.9 2D Modeling Report November 2024 Submittal, Memo to Matilija (MDERP) Project Team from Paul Jenkin, Matilija Coalition, December 13, 2024

Matilija Dam Ecosystem Restoration Project, Chronology Report, Ventura County Public Works Agency and Rincon Consultants, April 2023

Matilija Dam Removal, Sediment Transport, and Robles Diversion Mitigation Project, Dam Removal Concepts Evaluation Report, AECOM and Stillwater Sciences, January 2016

DRAFT 65% Design Report for Matilija Dam Removal, AECOM, March 2022

Predictability Assessment of Flushing Storm Event, Matilija Dam Removal 65 Percent Design Planning Study, AECOM, November 2022

Matilija Dam Removal 65% Design Subtask 2.2: Detailed Sediment Transport Modeling from Matilija Dam to Downstream to Ventura River Delta, Stillwater Sciences, February 2020

Matilija Dam Removal 65% Design Subtask 2.3: Hydraulic Studies to Determine 100-year Water Surface Elevations, Stillwater Sciences, February 2020

Matilija Dam Removal 65% Design Subtask 2.9: Hydraulic and Sediment Transport Modeling in SRH-2D, Stillwater Sciences, March 2025

Major, J. J., A. E. East, J. E. O’Connor, G. E. Grant, A. C. Wilcox, C. S. Magirl, M. J. Collins, and D. D. Tullos, 2017. Gravel-Bed Rivers: Processes and Disasters, First Edition, Chapter 13 Geomorphic Responses to Dam Removal in the United States – a Two-Decade Perspective.

Lessons Learned from Sediment Transport Model Predictions and Long-Term Postremoval Monitoring: Marmot Dam Removal Project on the Sandy River in Oregon, Bruce Orr 2014, Journal of Hydraulic Engineering. https://doi.org/10.1061/(ASCE)HY.1943-7900.0000894

More information and references are available at https://matilijadam.venturacounty.gov/

On this blog:

https://www.venturariver.org/search/label/Matilija%20Dam

Don't doubt the trout

On Saturday, March 1, the Ojai Valley Land Conservancy hosted an evening of music, film, and conversation at the historic Ventura River Steelhead Preserve.  The night began with a live performance by Par Avion, led by CalTrout Ambassadors Rebecca Ramirez and Bernard Yin, and a screening of Don't Doubt the Trout (directed by Ojai’s Isabela Zawistowska), which follows Rebecca and Bernard’s journey to understand Southern California’s endangered steelhead and how the experience deepens their connection to nature. The evening concluded with an outstanding panel of local conservation experts discussing fish, rivers, watersheds, and dam removal. 

PANELISTS 

Mark Capelli is with the National Marine Fisheries Service, where he is responsible for coordinating the development of recovery plans for the threatened and endangered steelhead trout of central and southern California (from Monterey to the U.S. Mexican border). He is the lead author of the "Southern California Steelhead Recovery Plan" and the most recent "2023 5-Year Review of Southern California Steelhead."

Rosi Dagit is Principal Conservation Biologist for the Resource Conservation District of the Santa Monica Mountains. She has initiated and coordinated numerous research, restoration and monitoring projects throughout the Santa Monica Mountains, starting with studies of Malibu Lagoon, Leo Carrillo State Beach, and moving inland to watershed level analysis and sensitive species monitoring. She recently led efforts to rescue tidewater gobies and steelhead trout in the Palisades Fire burn scar. Currently, Rosi leads the Topanga Creek Stream Team.

Paul Jenkin is the Ventura Campaign Coordinator for Surfrider Foundation, and founder of the Matilija Coalition. He serves on the Surfers Point Working Group, a multi-agency task force working to solve beach erosion problems near the mouth of the Ventura River. Paul has also served on the various technical work groups within the Matilija Dam Ecosystem Restoration Feasibility Study, and acts as the coordinator of the non-government efforts to remove Matilija Dam and restore the Ventura River watershed.

Russell Marlow is a CalTrout Senior Project Manager for the South Coast Region, leading the Ventura office. Russell previously worked for USFWS and the NPS as a biologist focused on native and endangered aquatic species before completing his graduate studies at South Dakota School of Mines studying aquatic macroinvertebrate assemblages. Russell’s project focus in the Ventura Office is on reconnecting critical habitat for the endangered southern steelhead through passage barrier removal or remediation.

Bernard Yin and Rebecca Ramirez serve as ambassadors for CalTrout, Patagonia Fly Fishing, conservation co-chairs for the Southern Sierra Fly Fishers Club, and perform music with their band, Par Avion.

More:

https://wildandscenicfilmfestival.org/film/dont-doubt-the-trout/

Riverbed changes after 2023 flood

The aerial photos shown in January 2023 Flood Overview illustrate how the flood of January 9, 2023 ripped out large areas of vegetation and "re-set" the floodplain.  In response to this 50 year flood, in many places the primary flow of the river shifted into historic channels within the wide floodplain.

This was the first major flood since the Thomas Fire of December, 2017.  As is typical for this watershed, erosion of thousands of acres of steep mountains following the fire introduced large amounts of sediment into the headwaters of the Ventura River.  This is documented in Post fire storm event, Jan 9 2018, Retrospective on the Thomas Fire, Ventura River post-fire sedimentation 2019, and Matilija canyon sedimentation.

Planning for the removal of Matilija dam has been underway since 1999, with the primary concern being predicting the fate of both the 9+ million cubic yards of sediment trapped upstream of the dam as well as the renewed sediment supply from Matilija Canyon once the dam is gone.  Planners and engineers have been relying on a variety of computer models to estimate changes in the riverbed with increased sediment supplies. Until now there has not been any real world data to groundtruth these predictions.    

A July 2024 report Post-Flooding Numerical Model Update and Review - Technical Memorandum generated for the Upper Ventura River Groundwater Sustainability Agency (UVRGSA) provides a hindsight analysis of channel changes following the 2023 flood based on LIDAR and satellite imagery.  The concern of the GSA was whether the changes in the riverbed would affect their groundwater computer model. Although the focus of this work was on calibration of the UVRGSA groundwater-surface water model, the document includes an interesting review of the changes in channel alignment and bed elevation summarized below.

Of interest is the conclusion that, 

"The changes in the channel characteristics generally depended on the location within the basin. The upstream areas were found to more likely increase in streambed elevations in response to sediment deposition and the downstream areas were more likely to observe decreases in streambed elevation in response to scour."

Of note: 

Upstream of Robles;

In the Kennedy hydrogeologic area, there was minimal change in the lateral position of the updated channel network’s position in comparison to the original channel network. However, the streambed elevation increased by approximately four feet on average (Figure 2.2a), indicating significant sediment deposition in this area. The accretion within the channel network increases moving downstream, with a maximum elevation difference of over eight feet occurring at the most-downstream reach. 

Ventura River preserve:

In the Robles North hydrogeologic area, the lateral position of the channel was minimally changed in comparison to the original channel in the northern half of this area. A new braid was formed in the southern half of this area (Figure 2.2b). Similar to the Kennedy zone, the overall channel within the Robles N zone increased in elevation by up to approximately three to five feet in some areas.

 

Within the Robles South hydrogeologic area, the lateral position of the updated channel network is generally in the same location as the original channel network (Figure 2.2c). However, the updated channel network is less tortuous than the original channel network. There is a small vertical change in the streambed elevation. On average, the updated channel network is approximately one foot lower than the original channel network. This zone represents a transition between sediment accretion in the upper portion of the basin and erosion in the lower portion of the basin. 

 

Live Oak Levee;

In the Santa Ana North hydrogeologic area, the updated channel shifted both westward and eastward in different reaches of the stream in addition to the formation of new braids (Figure 2.2d). The stream location begins westward of the original channel in the upstream areas and then crosses over eastward of the original channel until roughly rejoining the original channel. The updated channel network is consistently around two feet lower than the original channel network in this area, indicating the overall erosion of the streambed. 

Also of note, the “initial flush” from the Thomas Fire consisted of very fine sediment (ash and silt) that deposited in the riverbed following erosional flows up to 5,000 cfs (see Ventura River post-fire sedimentation 2019.)  The following winter the fine sediment was flushed and replaced by sand and gravel moving in flows up to 1500 cfs. (March rains.)   Then the big 2023 flood produced flows over 20,000 cfs which flushed the sand up onto the floodplain terraces and out to the beach, with the new channels almost entirely lined with cobble and boulders (including what some call “VW sized boulders”.)    

The 2024 storms had peak flows over 6,000 cfs which was enough to move this slug of sediment further downstream, and the active channel shifted dramatically again in response.  The image below is the 2023 main channel in Robles South that was abandoned in 2024 when sediment plugged its junction upstream.  

Former primary channel, Ventura River above Baldwin Rd, March 2024

Ventura River Flood flows, 2024 

Computer modeling for the Matilija Dam Ecosystem Restoration Project has predicted a sediment wave moving downstream following dam removal.  The results of this analysis demonstrate this effect, with a rise of 5-8 ft elevation in the riverbed seen moving into the upper reaches of the Ventura River following the 2023 flood.  It is likely that a followup analysis using LIDAR data after the 2024 floods would show this sediment moving further downstream, once again shifting the active channels and raising the riverbed in areas that were scoured in 2023.  

The Ventura River is dominated by the regional drought and flood cycles. This steep watershed has the natural capacity to transport huge volumes of sediment generated by the highly erodible sedimentary geology of the the transverse ranges of California.  Much of the Ventura River features a broad floodplain with many channels that shift in response to the ever changing flow of water and sediment.  For over 70 years Matilija Dam has trapped a large portion of the sediment yield from the upper watershed, and this deficit creates a net erosional environment downstream as seen in the downstream reaches of this study.  Although it's hard to quantify the volume, the pulse of sediment following the Thomas Fire is behaving much as predicted for the removal of Matilija Dam.  More on this to come...  

 

Reference:

Post-Flooding Numerical Model Update and Review - Technical Memorandum, Prepared for: Upper Ventura River Groundwater Agency, July 2024

https://uvrgroundwater.org/wp-content/uploads/2024/07/INTERA-UVR-channel-update-TM-2024-07-05_v3.pdf

On this blog:

Thomas Fire

Post fire storm event, Jan 9 2018

Retrospective on the Thomas Fire

Ventura River post-fire sedimentation 2019 

Matilija canyon sedimentation

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)

drone image showing reservoir sediment behind Matilija Dam, 5-28-2024
(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 base flows through the reservoir.   

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)

In the news:

Ventura County plans to repair Matilija Canyon Road which has been closed since the 2023 storms:

Mending Matilija: Temp repairs to damaged Matilija Canyon Road to get underway, with sights set on permanent fix, VCReporter Jan 15, 2026

The challenges of life in Matilija Canyon, Ojai Valley News, April 16, 2026

Plans are underway for full repair of the road. “Given how expensive it is to work back there, it took us some time to get the budget,” LaVere said. “The permanent repairs are actually taking place later this year, which we think are going to mitigate a lot of these issues.”

“But it’s not the permanent fix,” Lang said. “To have the river above the level of the road is not normal and will only be fixed by removing the dam as soon as possible to allow sediment to flow to the ocean naturally.”

Dam removal is still several years away, awaiting construction of additional downstream infrastructure and a new design for the Robles Fish Passage Facility, to manage the sediment stored behind the dam.

(The County has determined they need $500,000 to study how dam removal will impact Matilija Canyon Road)