MANTUA
Two state sources disagree about whether it is safe to get in
A local health department posted a danger advisory in May 2026 and three outlets reported one in July. The state page for this reservoir shows an all clear and has not been updated since June 2024.
This is the most important thing on this page and it is not settled, so read the whole of it before deciding anything.
| Source | What it says | Dated |
|---|---|---|
| Bear River Health Department, the local health authority | "Mantua Reservoir: Danger Advisory" | 27 May 2026 |
| Three news outlets | A danger advisory, the highest level | 8 and 9 July 2026 |
| Utah's environmental department, on its own page for this reservoir | "There are no current health advisories for this water body" | page last updated 19 June 2024 |
The health department notice is easy to miss even when you find it, because its headline reads "HABS WARNING Advisory" while the Danger line inside it belongs to this reservoir and the Warning belongs to a different water on the same notice .
This page cannot tell you whether that advisory is in force today. No rescind notice was found anywhere. Check with the Bear River Health Department before you or a dog get into this water, and do not rely on the state page, which is the one a search will hand you first.
The three news reports name three different bodies as the issuer: a threshold set by the state health department having been exceeded , the Bear River Health Department issuing it , and the Division of Water Quality issuing it . Those are not necessarily in conflict, because sampling, threshold setting and posting are different jobs. No source retrieved here says how they divide, so this page does not reconcile them and prints all three.
Not a Reclamation project. Reclamation's list of its Utah facilities does not include this reservoir . The owner is a city .
| Field | Value |
|---|---|
| Owner type | Local Government, and the state file gives the owner as Brigham City |
| Regulator | Utah Division of Water Rights, federally regulated No |
| Completed | 1961, earth, no recorded modification |
| Dimensions | 29 feet high and 11,140 feet long. That is a little over two miles of embankment holding back twenty nine feet of water |
| Hazard potential | High |
| Last inspected | 16 August 2023, on a one year cycle |
| Condition assessment | Satisfactory, dated 1 January 1994 |
The federal record's own vintage matters here. Its data updated field reads 28 March 2024 . A more recent state inspection may well exist, and this page does not claim none has happened since 2023.
Why the state file is not quoted for the newer dates. Utah publishes its dam inventory as a flat list, and that list drops empty cells rather than leaving them blank . Its opening block lines up cleanly and gives the owner, the stream and the regulator. From the dimensions onward the row cannot be aligned to its own headers without inventing at least two missing cells, and the copy this build retrieved stopped alphabetically before this dam. So two dates in that file that look like inspection dates are not published here, because this build cannot say which column either belongs to.
The dam is not on Box Elder Creek. The federal inventory files it on Big Creek , and the state watershed plan is more specific still :
"The two major inflows to the reservoir are Dam Creek and Maple Creek." And: "The outlet of the reservoir forms Big Creek."
And there are two Mantuas. The federal names authority carries an incorporated Town of Mantua in Box Elder County, Utah, and an incorporated Village of Mantua in Portage County, Ohio . The federal water body query used to draw the outline on this page returns both this reservoir and Mantua Swamp, Ohio, in the same response . Any search for an incident at this water will return the other state, and Box Elder is also a town in South Dakota.
The hatchery, the phosphorus, and fourteen feet of water
A state hatchery producing about a third of Utah's stocked trout is also the largest permitted phosphorus source to the eutrophic reservoir it sits beside.
Both halves of this are stated by state agencies, and they are not stated in the same document.
| The wildlife division says | The water quality division says |
|---|---|
| The hatchery was "first constructed in 1910" and produces "roughly 6 million trout eggs a year, roughly 30% of the total trout stocked in Utah" | "The Mantua Fish Hatchery is a significant contributor of nutrients to the Reservoir, adding an estimated 304.4 kg/y TP (31% of total load)" |
What is worth reading is the combination. Phosphorus is the pollutant this reservoir is impaired for, and the largest single permitted source of it is a state fish hatchery. Neither agency's document mentions the other's, and this page is putting them side by side rather than claiming any agency has drawn the connection.
The load figure is old. The watershed plan is dated 31 March 2000 , so the 304.4 kilogram figure is a quarter of a century old and the hatchery has been rebuilt since: the division completed a new facility there in February 2025, including a degassing tower, and the hatchery runs on groundwater rather than reservoir water . No newer load figure was found, so the old one is printed with its date attached and nothing is inferred about the present.
The state watershed plan is unambiguous about the condition of this water :
"The Clean Lakes Study verified that Mantua Reservoir is eutrophic and phosphorus is the nutrient limiting algae and aquatic plant growth."
| Measure | Published |
|---|---|
| Impaired for | exceedances of the pH, dissolved oxygen and total phosphorus criteria |
| Protected uses | Class 2B secondary contact recreation, Class 3A cold water game fish and aquatic life, Class 4 agricultural uses |
| Surface area | "224 ha (554 acres)" |
| Mean depth | 4.3 metres, which is about fourteen feet |
The plan is dated 31 March 2000. That is the newest agency document this build found giving this reservoir a depth or an area at all. No maximum depth is published in it, and none was found anywhere else.
Both dam records leave the surface area field empty . So the 554 acres on this page comes from the watershed plan and from nowhere else, and no shoreline length and no maximum depth is published by anyone.
The shape in the header is traced from the Fathom print artwork and checked against the federal water body polygon .
| Measure | Traced | Federal | Gap |
|---|---|---|---|
| Width against length | 0.9240 | 0.8836 | 4.57% |
| Share of its bounding box | 0.683 | 0.674 | 1.3% |
| Area, stretched on both axes | 522.6 acres | 515.4 acres | 1.41% over |
But the polygon computes to 515 acres against the watershed plan's 554 , and it carries a null elevation , so it cannot be checked against a working pool. Seven percent short with no elevation stamp reads as a low pool snapshot, so this page publishes 554 acres and not 515.
About a mile and a sixth long and just over a mile wide, from the federal polygon. The polygon also carries a small island: the response has a second ring, which the area calculation subtracts .
Where everything is
Seven places, two of which are in Ohio.
The campground beside this water is Box Elder Campground, run by the Uinta-Wasatch-Cache National Forest's Logan Ranger District .
| Published | Not published |
|---|---|
| Open 4/24 to 10/4 | Number of sites |
| Potable water available | Fees, which the page defers to the booking service for |
| Latitude and longitude, which is why the pin above is exact | Any horsepower or wake restriction |
It is not a state park, checked against the full Utah roster of forty six rather than against a sub page . No boat ramp, no horsepower limit and no wake restriction was found in any agency source read in this build, so none is stated here.
Every other page in this series carries at least one live federal number for the water itself. This one carries none, because none exists.
| What is missing | Why |
|---|---|
| Reservoir storage | No soil conservation service station exists for this reservoir. A search of every Utah station on that network returned no match |
| Stream flow | The only gauge ever placed on this water, Box Elder Creek at Mantua, ran 1 October 1960 to 29 September 1963 and then stopped |
So the live strip at the top of this page has one working card and it is the weather. The other three say what is missing and why, which is the honest version of a dashboard for a water nobody measures.
That absence is worth holding next to the advisory question. A reservoir with a documented bloom history, a danger advisory this summer, and a state monitoring page two years out of date is also a reservoir with no public instrument on it of any kind.
Eight fish from one survey, and a ninth that is only a proposal
The longest evidenced species list in this series, all of it from a single electrofishing survey. Walleye is named here only in a proposed rule, so it is written and not drawn.
Bluegill
The division's 2023 survey found bluegill were "the most abundant species by number caught (58.9%)" . The reputation this reservoir has for bluegill is not angler folklore: it is the top line of an agency electrofishing survey.
58.9% by numberLargemouth bass
The same survey found largemouth bass were "the most abundant species by weight (46.6%)" . Most abundant by number and most abundant by weight are two different fish here, which is what a panfish reservoir with a healthy predator looks like.
46.6% by weightYellow perch
Caught in the 2023 survey . Also the reason this water appears in the fishing guidebook at all: it is on the page 12 list of waters where dead yellow perch may be used as bait .
2023 surveyGreen sunfish
Caught in the 2023 survey . Drawn separately from bluegill because the division counted them separately, and they are easy to confuse in hand.
2023 surveyBlack bullhead
Caught in the 2023 survey . One of two catfish family species the division recorded here.
2023 surveyChannel catfish
Caught in the 2023 survey . The division names it and gives no abundance figure for it, so none is given here.
2023 surveyRainbow trout
Caught in the 2023 survey . Worth noticing given what sits on the shore: the hatchery beside this reservoir produces about thirty percent of the trout stocked in Utah .
2023 surveyFathead minnow
Caught in the 2023 survey . A forage species, and the only fish on this page nobody fishes for.
2023 surveyThis is the longest evidenced fish list in this series and the only one that comes from a single survey. Utah's wildlife division sampled this reservoir by boat electrofishing on 11 May 2023 and named eight species . All eight are drawn. Nothing else is.
The two flagged fish are flagged because the report says so. Bluegill at 58.9 percent by number and largemouth bass at 46.6 percent by weight . The reservoir's reputation as a bluegill and bass water turns out to match what the division actually caught, which this series has not often been able to say.
What the report does not give. No abundance figure for the other six species, no length or weight distributions reproduced here, and no statement about a population trend. Only one report exists: addresses for 2022, 2024 and 2025 all return errors, so there is a single year of survey evidence and no series.
There is a ninth species associated with this reservoir and it is deliberately not drawn above.
On 12 August 2026 the wildlife division published proposed regulation changes including this line, verbatim :
"Adding no limit on walleye (an illegal introduction) at Mantua Reservoir and requiring all walleye caught to be killed."
And it is a proposal, not a rule. The same release is explicit that the division is "seeking the public's feedback on the proposals", and the changes would take effect in 2027 and 2028 if approved . So this page draws eight surveyed fish and writes the ninth as a sentence, because a proposed rule is not a survey.
The same release proposes cutting the bluegill limit here from 50 to 15 . That is also a proposal. Anyone fishing this reservoir now is under the statewide limits, because this water has no entry in the current guidebook at all.
Mantua Reservoir has no waterbody entry in Utah's fishing guidebook. Not in the 2026 edition and not in the 2023 one . Statewide limits apply.
The absence was checked four ways: a literal string search, a full transcription of the printed run of entries where it would sit, an alphabetical order test, and a separately issued existence question. The alphabetical test settles it: the entries run Manning Meadow then McGath, and Mantua would sort between them.
Blue Ribbon: not designated. The division's Blue Ribbon page describes the programme and prints no list , and its most recent funding release, dated 15 July 2026, states there are 41 waterbodies including two mountain areas, names four project waters, and does not name this one .
Nothing on this page came from the division's fishing application. That application builds its content in the browser, so the served document contains no waterbody data for any water, and a search engine's stored title of it is not a source. That rule exists because this series published a Blue Ribbon claim off exactly such a title once and had to withdraw it.
What to watch for
A fish kill the wildlife division recorded in one line, no incident found in searchable press, and a shallow eutrophic reservoir that blooms hard enough to close.
The clearest sentence about what has actually happened in this water comes from the end of a fishery survey report, in the recommendations :
"Sample this water in 2024 to assess the impact of the fish kill that occurred in September 2023."
The state water quality board's minutes for October 2023 attribute that kill to treatment for harmful algal blooms. This build could not retrieve those minutes directly: the board's page is closed to automated reading. So the fish kill is stated here on the division's own authority and the algaecide connection is not, and the page says which is which rather than merging them.
The 2024 follow up survey the report recommends could not be found. Addresses for a 2024 report return errors, and the only report that exists for this water is the 2023 one. So whether the reservoir recovered is not answered by anything published.
No drowning, boating fatality, vehicle in the water, ice breakthrough, submerged debris recovery or dive operation at this reservoir was found in any source, agency or press.
The publishable sentence is "no incident found in searchable press". It is not "nothing has happened here."
Two gaps in this build, stated rather than hidden. Box Elder County sheriff records and Utah public safety records were not queried directly, and no source states which agency has primary water rescue responsibility at this reservoir. The town of Mantua has its own police department and the campground is federal, which are two more bodies in the same square mile.
The hazard at this reservoir is not an incident record. It is the water itself, and the state has described it in writing since 2000 .
| Condition | Source |
|---|---|
| Eutrophic, with phosphorus the limiting nutrient | the watershed plan |
| Impaired for pH, dissolved oxygen and total phosphorus | the watershed plan |
| Mean depth 4.3 metres across 554 acres | the watershed plan |
| Danger advisory for a harmful algal bloom | the local health department, 27 May 2026 |
| A fish kill in September 2023 | the wildlife division |
Cyanobacteria affect dogs more than people, because dogs drink the water and lick their coats, and the health department's notice says to keep dogs out . Check the health department before going in.
And the classification is 2B , which the watershed plan glosses as secondary contact recreation. That is a designation setting which standards apply and it is not a statement that the water is unsafe. It is the context worth having at a reservoir where the practical question every summer is whether it is safe to swim.
A danger advisory was posted for this reservoir in May 2026 and the state page for the same water still shows an all clear from June 2024.
Fathom Restoration is a Utah 501(c)(3) that runs volunteer dive cleanups and is built to pull what does not belong out of Utah water, statewide.
Donate to Protect MantuaEvery figure on this page is quoted from the source named beside it.
Sources
How these were checked
Numbered sources are agencies. The three lettered sources are news outlets, and they are here for one reason: they are the only sources reporting the July 2026 danger advisory, and all three attribute it to a different body. Where sources disagree, all of them are printed: that happens over whether an advisory is currently in force, over who issued it, over the surface area, and over which creek this reservoir is on. Two method notes are specific to this page. First, a search engine's stored title for the state monitoring page reads "Warning Advisory"; the page itself, fetched, reads "There are no current health advisories for this water body". The fetch governs, and a search result is not a source. Second, Utah's dam inventory is published as a flat list that drops empty cells, so its columns cannot be aligned past the opening block. Two date values in that row are not printed here, because this build cannot establish which column either belongs to.National Inventory of Dams, federal identifier UT00196, Mantua. County Box Elder, owner type Local Government, state regulated Yes by the Utah Division of Water Rights, federally regulated No, completed 1961, height 29 feet, length 11,140 feet, maximum storage 10,450 acre feet, normal storage 7,560, drainage area 9 square miles, surface area field empty, hazard potential High, condition assessment Satisfactory dated 1 January 1994, last inspected 16 August 2023, inspection frequency one year. The river field reads "Big Creek"
Mantua Reservoir total maximum daily load, 31 March 2000. Publishes the surface area as "224 ha (554 acres)" and a mean depth of 4.3 metres, names the two major inflows as Dam Creek and Maple Creek, states that the outlet forms Big Creek, records exceedances of the pH, dissolved oxygen and total phosphorus criteria, and gives the fish hatchery load as 304.4 kilograms a year of total phosphorus, 31 percent of the total
Mantua Reservoir recreational water quality monitoring, the state page for this water. Retrieved 14 August 2026, when it displayed "There are no current health advisories for this water body" and its most recent update was dated 19 June 2024
Harmful algal bloom advisory notice, 27 May 2026. Headed "HABS WARNING Advisory" and listing "Willard Bay Pond: Warning Advisory" and "Mantua Reservoir: Danger Advisory", with guidance not to drink the water, not to swim, wade or ski, and to keep dogs out
Recreational water quality. The statewide landing page. It explains the three advisory levels, names no waterbody and prints no date
Mantua Reservoir fishery monitoring report. Author Taylor R. Stewart, sampled 11 May 2023 by boat electrofishing. Names eight species, records bluegill as the most abundant by number at 58.9 percent and largemouth bass as the most abundant by weight at 46.6 percent, and recommends sampling in 2024 to assess the impact of the fish kill that occurred in September 2023
Proposed changes to fishing regulations, 12 August 2026. Proposes reducing the daily bluegill limit at this reservoir from 50 to 15, and adding no limit on walleye, described as an illegal introduction, with all walleye caught required to be killed. These are proposals for 2027 and 2028, not current rules
New facility completed at the Mantua Fish Hatchery, 20 February 2025. States that the hatchery was first constructed in 1910 and produces roughly 6 million trout eggs a year, roughly 30 percent of the total trout stocked in Utah, and that it runs on groundwater
2026 Utah Fishing Guidebook. This reservoir has no waterbody entry in Rules for Specific Waters. The only appearance of the name is on page 12, in the list of waters where dead yellow perch may be used as bait
2023 Utah Fishing Guidebook. The same absence, three years earlier
National Hydrography Dataset. Mantua Reservoir, permanent identifier 120030650, published at 2.0830 square kilometres with a null elevation and reach code 16010204001874. The same query also returns Mantua Swamp in Ohio
Geographic Names Information System. Two incorporated Mantuas exist in the United States: the Town of Mantua in Box Elder County, Utah, and the Village of Mantua in Portage County, Ohio
Box Elder Creek at Mantua, site 10126400. Daily discharge from 1 October 1960 to 29 September 1963, 1,094 values, and five peak flow records to 1 February 1963. Nothing since
Utah dam safety, the state inventory list. Carries the owner as Brigham City and the stream as Big Creek. The list is served as a flat file that drops empty cells, so its columns cannot be aligned positionally past the opening block, and the copy this build retrieved was truncated alphabetically before M
Box Elder Campground, Logan Ranger District. Open 4/24 to 10/4, potable water available, and describing Mantua Reservoir as a short drive from the campground rather than as part of the site
Blue Ribbon fisheries programme allocates over 446 thousand dollars, 15 July 2026. States that there are currently 41 waterbodies including two mountain areas, names four project waters, and does not name this reservoir
Blue Ribbon fisheries. The programme and its criteria, with no list and no count on the page, and no mention of this reservoir
Utah state parks. The full roster runs to forty six and this reservoir is not on it
Utah facilities. Reclamation lists its Utah dams, powerplants and projects, and Mantua is not among them
Point forecast over the reservoir. Forecast office Salt Lake City
Algal danger advisory issued for Mantua Reservoir, 8 July 2026. Reports a Danger Advisory and states that a threshold established by the Utah Department of Health and Human Services has been exceeded
Danger warning issued for toxins at Mantua Reservoir, 8 July 2026. Reports a high level danger warning issued by the Bear River Health Department
Daily news, 9 July 2026. States that the Utah Division of Water Quality issued a danger advisory, described as the highest advisory level for these blooms
Two state sources disagree about whether there is an advisory here
Read this before getting into the water, and then ask the health department.
The Bear River Health Department, 27 May 2026. A notice listing "Mantua Reservoir: Danger Advisory" . Its guidance: do not drink the water, do not swim, wade or ski, keep dogs out of the water, and if you fish, discard skin and guts. The notice's own headline reads "HABS WARNING Advisory", because the Warning belongs to a different water listed on the same page. The Danger line is this reservoir's.
Three news outlets, 8 and 9 July 2026. All three report a danger advisory, the highest level, and all three name a different body as issuing it: a state health department threshold being exceeded , the Bear River Health Department , and the Division of Water Quality .
Utah's environmental department, retrieved 14 August 2026. Its page for this reservoir displays "There are no current health advisories for this water body" . Its two dated entries are from 5 June 2024 and 19 June 2024, both reporting no visible signs of a bloom. There is nothing on it from 2025 or 2026 at all.
What this page will not do. It will not tell you there is a danger advisory in force today, because no rescind notice was found and no source establishes the current state. It will not tell you the water is clear either, because the page saying so has not been updated in over two years. It will tell you that the state page is the one a search returns first and it is the stale one.
And a search engine made the same mistake. A search result for that state page carries the title "Warning Advisory". The page, fetched, says the opposite. A stored title is not a source, in either direction.