Potholes Supplemental Feed Route

Mitigated Determination of Non-Significance (MDNS) Issued

On May 14, 2009, Ecology issued an MDNS determination under the State Environmental Policy Act (SEPA) for Phase 2 of the Potholes Supplemental Feed Route project. After studying potential impacts to the environment, Ecology determined that with appropriate mitigation, there will be no probable significant adverse environmental impacts.

Phase 2 MDNS

Phase 1 MDNS

Overview

In the late 1970s, the Bureau determined that a feed route to Potholes Reservoir was necessary to ensure a reliable water supply for the South Columbia Basin Irrigation District.

Aerial photo of Potholes Reservoir

The Columbia Basin Project (CBP) was designed to capture return flows from irrigation on the northern half of the project for use in the southern half of the project. However, irrigation in the north half does not yet provide enough return flow to fully supply the south half.

To correct this problem, a feed route was developed to move water from Banks Lake to Potholes Reservoir. The feed route transports water through the Main Canal to the split, then south through the East Low Canal (ELC) to Rocky Coulee Wasteway. The water is then discharged into Upper Crab Creek near the north end of Moses Lake. From this point, the water moves through Moses Lake and into Potholes Reservoir at the Moses Lake outlet structure. Feeding is done early and late in the irrigation season when demand for irrigation water is low. At these times, the “unused” capacity in the ELC is used to carry feed water to Potholes Reservoir.

This feed route solved the immediate problem in 1980 and is still used today. However, the ability of this route to meet needs has lessened over time. Improvements in irrigation efficiency in the northern half of the project have led to even lower return flows and an increased need for feed to supply the southern half. Demand has also changed. Block 26 was added to the Potholes system in 1984 and the East Columbia Basin Irrigation District (ECBID) Supplement No. 1 to the Master Water Service Contract allowed for additional use out of the ELC. As a result, the demand on Potholes is greater, and the amount of “unused” capacity in the ELC has declined. These factors and a need for system reliability have led to the need for a supplemental feed route (USBR, 2007 E.A.).

Crab Creek, Billy Clapp Lake & Frenchman Hills Wasteway

Environmental Assessment

Since 2005, Ecology has partnered with the Bureau and invested $2.1 million to study the need for a supplemental feed route. In August 2007, the Bureau issued a final Environmental Assessment (EA) and Finding of No Significant Impact for the Potholes Reservoir Supplemental Feed Route.

Alternatives Considered
The draft EA evaluated four alternatives for conveying the waters: “No Action,” Crab Creek Perennial (year-round use), Crab Creek Ephemeral (intermittent use), and Frenchmen Hills. During the comment period, the WDFW requested that, for purposes of fish and wildlife management, a Crab Creek option be developed that, while primarily used year round, included the option of occasionally providing only spring flows. Operating the Crab Creek feed route intermittently would be done to limit the spread of invasive species.

To accommodate this request, the Crab Creek alternatives were combined using the flow targets from the Crab Creek Perennial alternative: 500 cfs in the spring and 100 cfs for the remainder of the year in those years when flows are year round. Under this new alternative, less water can be fed down Crab Creek in the years when it would be operated only in the spring compared to the EA’s Crab Creek Ephemeral alternative that had a spring flow target of 650 cfs.

 

To accommodate this shortfall in years when the Crab Creek route would be operated in the spring only, the Frenchmen Hills route was added to the combined Crab Creek alternative. With the Frenchmen Hills route included, the new alternative could feed almost 80,000 ac-ft in years when Crab Creek is used only seasonally. Therefore, two alternatives, the No Action alternative and the combined Crab Creek and Frenchman Hills Wasteway alternative, were considered and evaluated in the final EA.

The Bureau identified Crab Creek and Frenchman Hills Wasteway as the preferred alternative. This alternative would release feed water from Billy Clapp Reservoir through the four-by-four-foot outlet into Brook Lake, a natural water body within the Crab Creek channel. Crab Creek would then convey the water into Moses Lake and Potholes Reservoir. Water would also be released from Billy Clapp Reservoir via the Main Canal and West Canal, into the Frenchman Hills Wasteway, and then into Potholes Reservoir (Bureau, 2007 EA). Implementation of this preferred alternative is expected to occur in 2008.

More Information

Frenchman Hills Wasteway improvements