Does the long earth embankment around the southern end of Mud Lake count as a 'dam' under Idaho's Dam Safety Act, and could the state be sued if the Water Resource Board failed to regulate it?
Plain-English summary
The Director of the Idaho Department of Water Resources asked whether the 13-mile, 10-foot-tall earth embankment that surrounds the southern edge of Mud Lake (in northern Jefferson County) is a "dam" for purposes of the Idaho Dam Safety Act, and whether the Idaho Water Resource Board would be liable if it exempted the embankment from regulation. He also asked whether having the surrounding landowners and water-right holders accept responsibility for the embankment as a dike (rather than a dam) would change the analysis.
The AG concluded yes: the Mud Lake embankment is a dam under Idaho Code § 42-1711(b), because it stores more than 50 acre-feet of water (the actual storage capacity is about 37,930 acre-feet at eight feet of water against the embankment). The statutory definition is disjunctive: an artificial barrier is a dam if it is either at least 10 feet tall or if it stores at least 50 acre-feet at maximum storage elevation, unless an exception applies. None of the 1987 amendments' five new exceptions covered Mud Lake.
The AG flagged a wrinkle. In Marty v. State, the Jefferson County district court had recently ruled that "Mud Lake must be considered as a natural as opposed to an artificially created body of water so far as the rules of law and rights of the public or of individuals are concerned." That made the statutory definition ambiguous as applied to Mud Lake's embankment: did the legislature intend the Department to regulate man-made structures as dams under the Dam Safety Act when those structures had acquired the attributes of natural embankments for tort-law purposes? The AG worked through the legislative history of the 1987 amendments and concluded the embankment did not fit any of the five exceptions Congress added in 1987, and the legislature's intent was to continue concentrating dam-safety oversight on dams that "pose a threat to the public or could cause extensive property damage." The Mud Lake embankment plainly did, since failure of a 13-mile, 10-foot embankment holding back nearly 38,000 acre-feet of water would be catastrophic.
Idaho Code § 42-1710 mandates the regulation of all dams. The statute provides discretion to determine the degree of regulation, but no discretion to exempt dams from regulation entirely. If the Board adopted regulations that violated this statutory duty, the Board could be liable for personal or property damage caused as a direct result of the violation. None of the immunity provisions of Idaho Code § 6-904 or § 42-1717 would shield the Board from liability for that violation.
On the dike-versus-dam question, the AG concluded that having the landowners or water-right holders accept responsibility for the embankment as a dike would not eliminate the Board's liability, because the statutory regulation requirement is keyed to whether the structure meets the dam definition, not on private parties' characterization of it.
Currency note
This opinion was issued in 1988. Subsequent statutory amendments, court decisions, or later AG opinions may have changed the analysis. Treat this page as historical context, not current legal advice. Verify current law before relying on any specific rule, deadline, or remedy mentioned here.
Background and statutory framework
The Idaho Dam Safety Act (Idaho Code § 42-1709 et seq.) gave the Idaho Water Resource Board regulatory authority over dams. Idaho Code § 42-1711(b) defined a dam in part as:
"Dam" means any artificial barrier, together with appurtenant works, constructed for the purpose of storing water or that stores water, which is ten (10) feet or more in height from the natural bed of the stream or watercourse at the downstream toe of the barrier, as determined by the department, or from the lowest elevation of the outside limit of the barrier, if it is not across a stream channel or watercourse, to the maximum storage elevation, or has or will have an impounding capacity at maximum storage elevation of fifty (50) acre feet or more.
The 1987 amendments added five categories of exemption: barriers in low-risk areas under specified height or storage limits, barriers in canals used to raise/lower or divert water, fills or structures designed primarily for highway or railroad traffic, and fills or retaining dikes for retention and treatment of municipal, livestock, or domestic wastes. The Mud Lake embankment did not fit any of the five categories.
The factual setting was unusual. Mud Lake is a natural lake in a depressed basin with no natural drainage outlet. Beginning in the 1920s, settlers built dikes around portions of the lake to reclaim land. Over time the dikes were linked together to form a 13-mile crescent-shaped embankment, about 10 feet high, with average storage capacity of 37,930 acre-feet. The embankment changed the lake's shape and storage capacity. The state district court in Marty v. State held that for tort-law purposes Mud Lake must be considered a natural water body, even with the embankment. That created an interpretive question: did the same characterization apply for Dam Safety Act purposes?
The AG concluded no. The Dam Safety Act focused on physical risk, and the Mud Lake embankment plainly posed a risk to public safety and property if it failed. The legislative intent reflected in the 1987 amendments was to continue regulating dams that posed real risk while exempting small or low-risk structures. The Mud Lake embankment was not a small or low-risk structure.
Common questions
Is the Mud Lake embankment a "dam" under Idaho's Dam Safety Act?
Yes. Idaho Code § 42-1711(b) defines a dam disjunctively: an artificial barrier is a dam if it is at least 10 feet tall or stores at least 50 acre-feet at maximum storage. The Mud Lake embankment stores about 37,930 acre-feet, well over the threshold. None of the 1987 exemptions covered it.
Did the Marty v. State district court ruling change that?
No. Marty held that for tort-law purposes Mud Lake must be considered a natural body of water. The AG read that as a private-rights ruling about whether public-trust principles apply to a particular body of water, not as a ruling that affects the statutory definition of "dam" under the Dam Safety Act. The Dam Safety Act focuses on physical risk; Marty's framing did not change the fact that the embankment is artificially constructed and stores water.
What's the Board's regulatory duty?
Idaho Code § 42-1710 mandates regulation of all dams. The statute provides discretion to choose how strict the regulation should be, but not to exempt a dam entirely. If the Board adopted regulations that violated that mandate (for example, by purporting to exempt the Mud Lake embankment), the Board could be liable for damage caused by the violation.
Can the Board claim sovereign immunity for failure to regulate?
The AG concluded no, with respect to violations of the statutory duty in § 42-1710. Idaho Code § 6-904 contains the Tort Claims Act exceptions, and § 42-1717 contains additional dam-safety-related immunity. Neither would shield the Board from liability for adopting regulations that violated the statutory duty itself.
What if the surrounding landowners agree to treat the embankment as a dike?
That doesn't help. The statutory definition turns on objective factors (height, storage capacity, artificial-barrier character), not on the parties' characterization. The Board's liability for failing to regulate stems from the statutory duty, not from how private parties classify the structure.
Citations
Idaho statutes: Idaho Code §§ 6-904, 42-1709 et seq. (Idaho Dam Safety Act), 42-1710, 42-1711(b), 42-1717; Act of March 25, 1987, ch. 98, 1987 Idaho Sess. Laws 192.
Idaho cases: Marty v. State, Jefferson County Civil No. I-3504 (Dist. Ct. December 17, 1987); Summers v. Dooley, 94 Idaho 87, 481 P.2d 318 (1971); St. Benedict's Hospital v. County of Twin Falls, 107 Idaho 143, 686 P.2d 88 (App. 1984); Noble v. Glenns Ferry Bank, Ltd., 91 Idaho 364, 421 P.2d 444 (1966).
Source
- Landing page: https://www.ag.idaho.gov/office-resources/opinions/
- Original PDF: https://ag.idaho.gov/content/uploads/2018/04/OP88-02.pdf
Original opinion text
Full opinion text is preserved in the linked PDF. The opinion analyzed three questions about the Mud Lake embankment (a 13-mile crescent-shaped earth barrier built in the 1920s by linking individual landowner dikes) under the Idaho Dam Safety Act. The AG concluded: (1) the embankment is a dam under Idaho Code § 42-1711(b)'s disjunctive definition because it stores more than 50 acre-feet of water (about 37,930 acre-feet) and none of the 1987 amendments' five exemptions applies; (2) Idaho Code § 42-1710 mandates regulation of all dams, with discretion only as to degree, and the Water Resource Board could be liable for damage caused by adopting regulations violating that duty, neither Idaho Code § 6-904 nor § 42-1717 providing immunity for that violation; and (3) treating the embankment as a "dike" with private landowners assuming responsibility would not eliminate the Board's regulatory duty or its liability for breaching it. The AG addressed the Marty v. State (Dec. 1987) Jefferson County ruling that Mud Lake must be treated as a natural body of water for tort-law purposes, and read that ruling as not affecting the Dam Safety Act's regulatory definition.
DATED this 15th day of April, 1988.
JIM JONES
Attorney General
State of Idaho