Mercer Reservoir

Dallas Water Supply and Storage

Current Storage at Mercer Reservoir

The Rickreall (originally LaCreole) Creek Watershed, about 3.5 miles west of Dallas, supplies water for the Dallas Water System. The system has evolved from intakes on Rockhouse Creek, Applegate Creek, and Canyon Creek (tributaries to Rickreall Creek) in 1919, to the present dual intake system. In addition, water is stored behind an earthen dam about 4.5 miles upstream from the intake. Water is released from the dam when the natural stream flow is inadequate to meet the demand for water. The dam was constructed in 1959 to store 760 acre-feet (247 MG) of water. In 1972 the dam was raised to provide a total raw storage of 1,550 acre-feet (505 MG). Field survey data gathered in 1998 indicate that sedimentation in the impoundment had decreased total raw storage capacity to approximately 1,050 acre-feet (342 MG). Construction of flashboards completed in April 2001 added 215 acre-feet (70 MG) of spring/summer storage. Currently, the intake pumps are capable of delivering about 8.6 million gallons per day (mgd) to the water treatment plan.

Future Needs

Although the available storage in Mercer Reservoir remains sufficient for the average watershed yield well beyond 2040, analysis indicates that the City could face a shortfall should a 100-year drought occur. If the City's water demands increase as projected and a 100-year drought occurred, the available storage in Mercer Reservoir would be exhausted by the year 2028. The City is taking a proactive approach and is starting the process to expand our water storage to provide for at least 50 years of projected water demands under drought conditions.

On August 3, 2023, the City and Jacob's Engineering held a open house meeting for stakeholders to describe the need, proposed solution and next steps. A copy of the presentation from that meeting is available on the City's Document Directory.

Mercer Dam Q&A

Q. Are Earthen Dams outdated and dangerous?
A. Earthen dams are a type of embankment dam. These dams are constructed to hold water in a reservoir. Embankment dams are constructed from compacted soil and/or rock and are the most common type of dam in the U.S. They have been used for centuries and are still the preferred choice for many locations.

Some advantages for using an earthen dam are:

Ideal for construction in remote locations with limited access.
Material can be drawn from local resources (earthen materials), keeping sourcing costs low.
Suitable for construction in areas without a solid bedrock foundation.
Most economical option.

While dams have been being constructed for over 5,000 years, the last 100 years have seen the most significant contributions in dam engineering including understanding seismic forces on dams. Dams of all types require extensive engineering and geotechnical investigation for designing to withstand the forces of an earthquake.

Some of the seismic performance criteria utilized for earthen dams are:

Constructed with materials that do not easily liquefy and do not include sensitive clays.
Built and compacted to material densities capable to withstand an earthquake.
The slopes of the dam faces are 2.5:1 or flatter to disperse the weight of the reservoir waters.

Earthen dam performance is usually related to the nature of the soil used for the structure. By using materials that are seismically sound the structure will mimic the source material’s characteristics. A well-constructed earthen dam can significantly vibrate with an earthquake without damaging effects. These dams have better flexibility to accept the dynamic load due to earthquake force than concrete dams.

Q. What does it mean that Mercer Dam is labeled “High Hazard”?
A. Dams are classified by regulatory agencies based on their hazard potential or anticipated downstream consequences in the event of failure or mis-operation. The failure of a dam that is classified as “High Hazard-Potential” is anticipated to cause a loss of life and/or serious property damage. As of 2019, there are approximately 15,600 dams in the United States that are classified as high-hazard structures. The hazard potential classification does not reflect in any way on the current condition of the dam (i.e., safety, structural integrity, flood routing capacity).

Q. Why did the Condition Status of Mercer Dam move from “Fair” to “Poor”?
A. In our most recent inspection report by Oregon Water Resources Department dated January 28, 2022 it was noted that “This dam is well maintained and operated and is in Fair Condition”.

Mercer dam has only recently moved from “Fair” to “Poor” after a March 2022 study calculated the Probable Maximum Flood (PMF) would likely overtop the existing dam. A PMF is determined by calculating the highest probable amount of water that could collect and flow out of a drainage. PMFs are required to be used on flood calculations of High Hazard dams and use worst case scenarios for precipitation, temperature, soil absorption and snowpack to calculate the model storm event. This type of event is so unlikely to happen that it is somewhere in the 1 to 15,000-20,000-year probability. PMFs are typically estimated to be 4 to 5 times greater than the worst observed flood in a given drainage.

Q. Can’t you just dredge out the current reservoir easier than building a new dam?
A. Mercer Reservoir with its current dam has a maximum capacity of 1,500 Acre/Feet (AF). Due to erosion and silting, the current capacity is 1,200 AF. Dredging would effectively restore the reservoir to at most the original 1,500 AF. That volume will not be able to support Dallas’s service area much more than another decade or so. It is a fiscally responsible decision to focus on the development of a new dam with a reservoir capacity of 4,900 AF. This volume is projected to serve Dallas at its 2070 population projection during a 100-year drought event, with a volunteered additional release of 1 MGD for fish populations and other beneficial uses.

Q. Won’t building a larger dam make it harder for fish species to migrate?
A. Fish passage is always a concern with dams on fish bearing streams. On Rickreall Creek at Mercer Dam, upstream passage for Coho Salmon has been performed with a trap and haul program since 2009. Returning numbers have continued to increase, showing that the program is operating successfully. This type of upstream passage can be applied to many species of fish. The City’s existing dam is identified by ODFW as a critical barrier for fish passage, mainly due to its dependence on an upstream fish passage program and a deficient downstream waterway. There is a project in the works to improve downstream fish passage with the addition of splash pools on the downstream side of the spillway.

The design of the new dam will provide downstream fish passage and the City will continue to utilize a trap and haul program for all the required species.

Q. What are the benefits of expanding the capacity of Mercer Reservoir?
A. The construction of a new dam will have a myriad of benefits. Many will correct existing deficiencies while others benefit the water quality and quantity available in the Rickreall Creek.

Correcting existing deficiencies

Will be designed to safely pass the Probable Maximum Flood.
Fish Passage will be designed to support upstream and downstream migrations.
Design will follow current standards for Seismic Resiliency.
New dam will replace current structure that is nearing end of life.
A new dam will support gravel transport to the downstream side of the dam to enhance stream health and fish habitat.

Water Quality & Quantity

Meet community water needs into the future using PSU population projections for 2070.
A higher voluntary release will lower water temperatures during summer months.
Increased volume of water will enhance fish passage, spawning, rearing, and transport.
Increased volume of water will support agricultural water rights holders during the irrigation season.
Provide a potential water source for wildland firefighting use.