AB 639 amends Section 6003 of the California Water Code to add a targeted exclusion from the statutory definition of "dam" for certain agricultural weirs. The exclusion applies only when a barrier does not impound water above the top of a levee, the maximum storage behind the barrier leaves at least three feet of freeboard on that levee, and the structure qualifies as a "weir" under a federal flood control definition.
The bill lists nine specific weirs by name.
Practically, removing these structures from the "dam" definition takes them outside the Water Code provisions that trigger Department of Water Resources (DWR) supervision of construction, enlargement, alteration, repair, maintenance, operation, and removal. That narrows state-level regulatory reach for those named weirs, creates reliance on a federal definition, and raises measurement, enforcement, and liability questions for operators and downstream communities.
At a Glance
What It Does
The bill adds a new subdivision to §6003 excluding certain agricultural weirs from the statutory definition of "dam" when they do not impound water above a levee and provide at least three feet of freeboard. It limits the exclusion to nine weirs listed in the statute and defines "weir" by reference to 33 C.F.R. Part 208.10.
Who It Affects
Owners and operators of the nine named weirs—typically agricultural water delivery entities and local reclamation districts—will be directly affected, as will the Department of Water Resources, downstream property owners, and local emergency managers. Federal flood-control authorities may also be implicated because the weir definition depends on federal regulations.
Why It Matters
This is a narrow, structure-by-structure carve-out that removes state dam oversight for the listed facilities while leaving other similarly sized or functioning structures under DWR authority. The choice to use a static statutory list and a federal-reference definition creates legal and operational ambiguity about applicability, monitoring, and long-term risk allocation.
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What This Bill Actually Does
Section 6003 currently defines what counts as a dam for purposes of state supervision and safety regulation. AB 639 leaves the existing, long-standing height and storage volume exceptions in place (barriers ≤6 feet or storage ≤15 acre-feet) and adds one more narrow category: certain agricultural weirs that do not raise water above a levee and that maintain at least three feet of freeboard.
The bill then names which structures qualify and pins the technical meaning of "weir" to a federal flood-control rule.
If a structure meets the new exclusion, it ceases to be a "dam" under §6003. In practical terms that removes the statutory hook that gives DWR authority to supervise or require permits for construction, alteration, maintenance, operation, and removal under the state dam safety regime.
For the owners of the listed facilities, that reduces the need to coordinate those activities with DWR; it does not, however, itself create a separate state permitting program or safety standard for those weirs.The definition of "weir" in the bill is not purely descriptive; it requires that the structure "functions as part of a federal flood control system" as defined in 33 C.F.R. Part 208.10 and have mechanically or manually removable flashboards or gates.
Tying the exclusion to federal regulation means the state classification depends on whether the federal designation applies now and in the future — a point that could change how the exclusion is interpreted or challenged.Finally, the bill is a static, name-by-name exemption: only the nine structures listed in the statute are covered. That means any other similar weir elsewhere in the state remains a potential "dam" under §6003 unless separately exempted by future legislation or regulatory action.
Because the statute hinges on a freeboard measurement and on "maximum storage," it leaves open questions about when and how those measurements are made and who certifies compliance.
The Five Things You Need to Know
The bill amends Water Code §6003 to add a new exclusion for weirs that do not impound water above the top of a levee and that leave at least 3 feet of freeboard.
That exclusion is limited to nine specifically named facilities: Peoples Weir, Reynolds Weir, Last Chance Weir, Lemoore Diversion Weir, Island Weir, Crescent Weir, Stinson Weir, Empire Weir No. 1, and Empire Weir No. 2.
It defines "weir" as an agricultural water delivery structure with removable flashboards or gates that 'functions as part of a federal flood control system' under 33 C.F.R. Part 208.10.
Existing exemptions for barriers not exceeding 6 feet in height or storage not exceeding 15 acre-feet remain unchanged; this is an additional, narrower carve-out.
A structure that meets the exclusion is no longer a "dam" under §6003 and therefore falls outside the Water Code provisions that trigger DWR's statutory supervision of construction, alteration, maintenance, operation, and removal.
Section-by-Section Breakdown
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Retains the existing height and volume exceptions
Subsection (a) continues to exclude barriers ≤6 feet high and structures with storage ≤15 acre-feet from the definition of "dam." Keeping these clauses intact matters because AB 639 is additive: it does not replace or modify the preexisting mechanical thresholds that have long governed minor structures. Compliance officers and counsel should still treat those thresholds as the first line of exemption analysis.
New functional exclusion for weirs meeting a freeboard condition
This clause creates the core substantive test: a barrier that does not impound water above the levee top and whose maximum storage provides a minimum of three feet of freeboard on the levee will not be a "dam" if it is a qualifying weir. Practically, that turns an engineering measurement (freeboard at maximum storage) into the statutory trigger for exemption and shifts attention to who documents or verifies those measurements.
List of covered structures — a statutory whitelist
Rather than using geographic, functional, or agency criteria alone, the bill expressly names nine weirs that are eligible for the exclusion. Because the statute is prescriptive, adding or removing a structure from the list will require another legislative amendment; there is no administrative pathway inside this statute to expand the exemption to other facilities.
Definition of "weir" tied to federal flood-control regulation
The bill defines "weir" by reference to an agricultural delivery structure with removable flashboards or gates that 'functions as part of a federal flood control system' per 33 C.F.R. Part 208.10. That cross-reference imports federal criteria into state classification, which can complicate enforcement if federal designations change or if parties dispute whether a particular structure "functions as" part of the federal system.
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Explore Environment in Codify Search →Who Benefits and Who Bears the Cost
Every bill creates winners and losers. Here's who stands to gain and who bears the cost.
Who Benefits
- Operators and owners of the nine named weirs (e.g., local irrigation districts and agricultural water users): they avoid the procedural requirements and supervisory oversight that attach to state-recognized dams, which can lower permitting time and compliance costs for repairs and routine work.
- Private contractors performing maintenance or repairs on the listed structures: fewer DWR-imposed supervision requirements may reduce inspection bottlenecks, accelerate project timelines, and lower contractor administrative burdens.
- Local reclamation districts or county public works departments that manage levees adjacent to those weirs: increased flexibility to operate and modify these structures without seeking DWR review can simplify routine flood-control operations.
- Landowners immediately adjacent to the named weirs who rely on timely water-delivery repairs: reduced state oversight can speed fixes that maintain irrigation schedules and crop production.
Who Bears the Cost
- Downstream residents and property owners: removing state supervision could reduce independent oversight of safety standards, potentially increasing flood risk or delaying state-led inspections after incidents.
- Department of Water Resources: the exclusion shrinks the statutory perimeter of DWR authority and may lead to gaps in statewide dam data, complicating long-term monitoring and statewide hazard assessments.
- Local emergency response agencies and counties: if oversight gaps increase incident risk, local governments may face greater emergency response and recovery costs without corresponding state support.
- Insurers and lenders financing projects near the weirs: ambiguity about regulatory oversight and safety standards can increase perceived risk and raise premiums or lending costs for affected properties.
Key Issues
The Core Tension
The bill balances two legitimate objectives—reducing regulatory friction for routine agricultural water-delivery structures and preserving a consistent, statewide safety regime for barriers that pose flood risk—but it resolves that trade-off by narrowing state oversight for a small set of facilities, leaving open who will verify safety, and how to handle similar but unlisted weirs.
The statute turns an engineering judgment into a legal exemption without specifying procedures for measurement, certification, or periodic re-evaluation. The bill speaks of "maximum storage" and a minimum of three feet of freeboard but does not say whether those measurements are seasonal, event-based, or subject to calibration, nor does it designate a certifying authority.
That leaves operators, DWR, and courts to sort out how to apply the standard in practice.
Tying the exclusion to a federal regulatory definition (33 C.F.R. Part 208.10) reduces the state's need to restate technical criteria but creates dependency: if the federal designation of a system or the interpretation of that regulation changes, the state classification could shift without state legislative action.
Likewise, exempting a specific list of nine structures by statute creates a brittle solution — it solves the problem for the named facilities but offers no administrative path for similar structures to obtain the same treatment. Finally, removing DWR supervision does not erase private liability or other state and local legal obligations; it merely reallocates where risk is managed and may invite litigation over whether a facility truly falls outside the dam definition.
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