BACKGROUND

The Cache Creek Area Plan ("CCAP") was adopted by the Yolo County Board of Supervisors in 1996.  The CCAP consists of the Off-Channel Mining Plan ("OCMP") and the Cache Creek Resources Management Plan ("CCRMP").  The goals of the CCAP are protection of groundwater, preservation of agriculture, restoration of Cache Creek, and regulation of commercial mining.

The CCRMP (adopted August 20, 1996, amended August 15, 2002, and December 17, 2019) is an adaptive watershed restoration and management plan.  It eliminated in-channel commercial mining and established an improvement program for implementing ongoing projects related to channel stability and restoration of habitat within and along Lower Cache Creek.  The CCRMP provides a policy framework for the restoration of 14.5 miles of the creek and includes specific implementation standards.  The Cache Creek Improvement Program ("CCIP") is the implementation plan for the CCRMP and identifies specific categories of projects that include bank stabilization, channel maintenance, revegetation, and habitat restoration.

The CCIP is overseen by the Cache Creek Technical Advisory Committee ("Cache Creek TAC"), which is a three-person interdisciplinary group composed of a hydraulic engineer, a fluvial geomorphologist, and a riparian biologist.  The Cache Creek TAC was established to provide scientific and technical review and oversight for all projects conducted under the CCIP, as well as collect and evaluate scientific data on hydrologic, hydraulic, sediment transport, and biological conditions within the CCRMP area.  The Cache Creek TAC is currently staffed (via contracts held by the County) by the following subject-matter experts:

Section 2.2 of the CCIP requires that the Cache Creek TAC produce an annual report each year for the Yolo County Board of Supervisors.  The report is intended to: 1) document the previous year's monitoring results, field observations, and completed in-channel restoration or stabilization projects; 2) provide analysis of data and trends; and 3) identify recommendations and projects to be implemented the following year(s).  This report represents a critical component of the adaptive management of Lower Cache Creek.

The annual report subject to this staff report is for the 2023 Water Year, which covers the period of October 1, 2022, through September 30, 2023 (Attachment A).

SUMMARY OF 2023 WATER YEAR FINDINGS

Hydrologic and Water Quality Findings

The 2023 Water Year was generally fairly wet for the State of California, which recovered from previous drought years due largely to significant snowfall in the Sierra and a series of storms in January and March 2023. For Cache Creek, which is not affected by snow as is the greater Sacramento Valley, the water year was “above normal” to “wet.”  The largest precipitation event of the year occurred March 14-15 and generated flows of approximately 12,000 cubic feet per second (“cfs”) at Rumsey and 10,000 cfs at Yolo, which represents approximately a 2-year return interval peak. Two additional events in March produced flow events of 6,000 to 8,000 cfs, while a series of storms in January generated approximately five peak events between 4,000 to 9,000 cfs. These seven events represent approximately “average” annual peak flow events.  While individual flow event magnitudes were not remarkable in Water Year 2023, the number of events at, or just above, the annual flow event was notable.  Such flows are beneficial for ecosystem benefits, while not creating hazardous flood or erosion conditions.

Geomorphology Findings

Water Year 2023 was much wetter than the Critically Dry water years in 2021 and 2022 in the Cache Creek basin.  However, Water Year 2023 was quite different with respect to geomorphic conditions than the most recent wet water years in 2017 and 2019 because peak flow magnitudes were relatively low.  The peak flow at the Yolo USGS streamflow gage reached 9,936 cfs in March 2023, well below the threshold of 20,000 cfs required for significant sediment transport and channel change in Cache Creek.  Therefore, while delivery and transport of sediment to, and through, the CCRMP area in Water Year 2023 was the fifth highest over the last 19 years, channel change due to erosion, scour, and deposition was localized and mostly relatively minor.  The most obvious change in Lower Cache Creek geomorphic conditions after the peak flows was the combination of removed in-channel and riparian vegetation, and deposition of fresh sediment on channel bar surfaces.

Because of the relatively moderate channel change in Water Year 2023, the recommendations developed by the Cache Creek TAC did not change substantially from recommendations made in 2018, 2019, 2020, 2021, or 2022. One notable exception is the recommendation for conditions at Huff’s Corner at the downstream end of the CCRMP area.  High flows in Water Year 2023 coincided with recently excavated and unvegetated conditions from the Cache Creek TAC-recommended island removal project in this reach and caused significant erosion, scour, and deposition at the site.  While channel change like this is expected at newly constructed sites, conditions at Huff’s Corner should be closely monitored in Water Year 2024, especially if wet conditions return.  In addition, because Water Year 2023 did result in significant sediment transport, the TAC geomorphologist recommends that a comprehensive review of current recommendations be completed in Water Year 2024 after the high flow season and before the 2024 Creek Walk.  The purpose of this review is to focus Creek Walk observations on expected areas of significant change and need for revised recommendations.

Biological Resources Findings

The distribution, extent, and condition of native vegetation in 2023 was generally similar to conditions observed in 2022. Native vegetation recovery in areas previously impacted by off-highway vehicle (OHV) use was again noted in 2023, as was increasing herbaceous vegetation adjacent to pools created by intact beaver dams in some locations. There were four notable exceptions regarding the condition of native vegetation in 2023 compared to 2022: (1) further reductions of in-channel vegetation in the same areas noted in 2021–2022 due to winter 2022-2023 flows; (2) additional evidence of drought-stressed vegetation, although there was some vegetative regrowth in areas noted in 2021–2022; (3) significant negative impacts to mature native vegetation due to fire in several locations, and (4) minor loss of native trees in one location due to scour and bank erosion resulting from winter 2022-2023 flows.
 
Non-native and invasive plant species remain widespread along Lower Cache Creek, and continue to be one of greatest constraints to further recovery of native habitat.  As in recent years, arundo, tamarisk, Ravenna grass, and other non-native species were frequently observed and are potentially re-establishing and spreading in some locations.  As in 2022, some evidence of continued treatment of arundo and tamarisk was observed, although some plants were starting to resprout. Many additional non-native and invasive species (e.g., Himalayan blackberry, perennial pepperweed, and tree tobacco) remain common along Lower Cache Creek and should be prioritized for treatment and monitoring when and where feasible.  After treatment of non-native and invasive species, native woody and herbaceous species should be planted whenever possible to enhance habitat and reduce the potential for re-invasion.

 
Many common and special-status species of wildlife, invertebrates, and fish were again observed by the Cache Creek TAC, Cache Creek Conservancy staff, and volunteer observers during the 2023 Creek Walk.  Swainson’s hawks (State threatened) were observed in six of the seven reaches, and a occupied nest was observed in the Dunnigan Hills Reach. Bald eagles (State fully protected) were observed at two locations in the upstream portion of the Capay Reach, and a white-tailed kite (State fully protected) was observed in the Hoppin Reach.  Three active colonies of bank swallows (State threatened) were observed in the Hoppin Reach, and numerous other potential bank swallow colony sites were also observed starting in the Hungry Hollow Reach and downstream to the Hoppin Reach. A total of 48 unique bird species were observed in 2023 including acorn woodpecker, ash-throated flycatcher, blue grosbeak, Bullock’s oriole, great horned own, green heron, lesser nighthawk, marsh wren, red-tailed hawk, song sparrow, and yellow-headed blackbird.  Western pond turtles (State species of special concern) were observed in deeper pool in the Capay, Guesisosi, and Dunnigan Hills reaches. Some pools appeared to be the result of significantly increased beaver activity first observed in 2022 and attributed to reduced OHV activity. Beaver dams were frequently observed starting in the Guesisosi Reach and in other downstream reaches. Other wildlife species observed included Columbian black-tailed deer, black-tailed jackrabbit, California ground squirrel, coyote, garter snake, river otter, and fishes including bluegill, common carp, green sunfish, and largemouth bass.
 
Significant opportunities for habitat enhancement and restoration are essentially unchanged since 2019, including upland areas on the north bank from RM 26.8–27.8, the PG&E “Palisades” site (RM 26.8), Capay Open Space Park (RM 26.3), the Hayes “Bow-Tie” property (RM 20.0), the Millsap property (RM 18.5), the Moore Siphon repair site (RM 18.0), Wild Wings Open Space Park (RM 17.0), the Correll and Rodgers properties (RM 13.7), the Capay Organic creek frontage (RM 27.9) identified in 2019, and off-channel pits in the Dunnigan and Hoppin reaches (e.g., on the north bank from RM 15.0–15.4).  Based on 2017 and 2018 Creek Walk observations, the long-term resilience of revegetation and restoration projects within or adjacent to the active channel should be carefully considered prior to implementation, since such projects can be negatively impacted or completely removed by high flows. Passive restoration (e.g., streamflow enhancement and invasive species treatment) may be a more cost-effective approach for in-channel or near-channel locations subject to high flows.
 

SUMMARY OF 2023 WATER YEAR RECOMMENDATIONS

Hydrologic and Water Quality Recommendations

The CCTAC Hydraulic Engineer recommends the following:

Geomorphology Recommendations

As in previous years, geomorphology recommendations for Water Year 2023 are in three general categories: monitoring, evaluation, and implementation.

Monitoring is recommended at multiple sites including:

Monitoring at these locations should focus on lateral channel migration, sediment deposition, and erosion.  

The Cache Creek TAC Geomorphologist updated the recommendation for Huff’s Corner monitoring and continues to recommend previous evaluation and implementation actions as described below:

Biological Resources Recommendations

 

Recommendations regarding biological resources from the Cache Creek TAC Riparian Biologist are grouped into four general categories: native vegetation monitoring and management, habitat restoration, invasive species monitoring and management, and special-status species.

Implementation of the Cache Creek Area Plan includes coordination and collaboration with a multitude of stakeholders including creek-side landowners; aggregate producers through the Yolo Working Group of the California Construction and Industrial Materials Association; Cache Creek Conservancy; Yocha Dehe Wintun Nation; Yolo County Resource Conservation District; Yolo County Flood Control and Water Conservation District; Yolo Habitat Conservancy; Yolo Subbasin Groundwater Agency; and various state and federal agencies. 

Implementation of the CCAP is completely self-funded by revenue generated from fees collected through the Gravel Mining Fee Ordinance (see Title 10, Chapter 11 of the Yolo County Code of Ordinances).  The County receives a fee for every ton of gravel sold by the aggregate producers regulated under the program.  There is no budgetary impact associated with the recommendation actions contained in this staff report.