Twelve Mile Canyon Water Quality Study Summary
Background Information:
Water resources are critical to the economic, social, environmental and physical survival of all citizens in Central Utah. The San Pitch River watershed impacts citizens in Sanpete County, Juab County and Millard County. It is tributary to the Sevier River and feeds Yuba Reservoir, the primary water storage facility for Millard County. One of the primary sources of water for the San Pitch is runoff and snow melt that comes from Twelve Mile Canyon Creek and its tributaries. This watershed is critical to the San Pitch River, Sevier River, and all entities downstream of Yuba Reservoir. The surface water carries an extreme sediment load and significantly impacts the secondary systems of Gunnison City, Mayfield Town, and Centerfield Town, thereby increasing the demand on the culinary water systems. Yuba Reservoir is quickly filling with sediment, making sustainability a legitimate concern within the entire three-County region. Twelve Mile Canyon is a major contributor to the problem.
Geologic features within Twelve Mile Canyon are generally unstable. The upper layers consist of Flagstaff Limestone and the lower layer is North Horn Shale. The overlaying limestone is fractured, jointed, and has karst sinkholes near the top of the watershed. The underlying shale is impermeable, creating a failure plane as water penetrates the limestone layers above and ’Äúgreases’Äù the shale. Two slides are of primary concern but there are several other slides contributing to the problem.
Impacts to Water Quality and Regional Watershed:
During and immediately following the high water years of 1982-1984, multiple landslides and debris flows were either activated or reactivated, causing discharge of large amounts of sediment into the watershed. As the landslides and debris flows move downward, the newly exposed material at the toes of these features are eroded and taken into the streams. As discussed above, communities and irrigators then have to utilize this sediment-laden water. Please see the following photos.
Sediment is an extremely serious issue within the watershed. It reduces water efficiency, fills ponds, plugs pipes and sprinklers, ruins equipment, and costs several thousands of dollars per year to dredge ponds and fix piping systems. Annual costs and losses are estimated at nearly $500,000/year within the South Sanpete irrigation service areas alone. Total damages have been conservatively estimated by these irrigation companies at over $10 million since the 1983-1984 floods as shown on the following pages.
Pursuit of Solutions:
The Utah Division of Water Quality recognizes the concerns and has stepped forward with $150,000 for the Gunnison and Mayfield Irrigation Companies for Phase I of the project. Please note the list of the multiple agencies supporting and sponsoring the project in this packet. Additional funding is necessary to complete Phases II and III. The synopsis of the Phases is as follows:
- Phase I: Mapping and Data Gathering. Geotechnical, hydrogeologic, economic, aerial mapping, topographical, water quality and other existing data will be obtained. Funding is secured.
- Phase II: Data Evaluation and Alternatives Analysis. After the existing data is gathered, alternatives intended to address the problem will be generated. Phase II is estimated to cost a total of $300,000. A matching grant would be requested from the Community Impact Board (CIB). Approval of this request would be most certain as the CIB historically favors planning efforts, especially those with far-reaching impacts.
- Phase III: Final Design and Construction. The preferred alternative will be designed and implemented. Additional monies would be sought for this phase.
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