Overview
Lower Monumental Lock and Dam is a concrete run-of-the-river hydroelectric facility situated on the lower Snake River in the northwest United States. Located in southeast Washington, the structure bridges Franklin and Walla Walla counties. It is positioned six miles (10 km) south of Kahlotus and 43 miles (70 km) north of Walla Walla. The project is operated by the United States Army Corps of Engineers and has been operational since its commissioning in 1969.
The facility generates 810 MW of capacity, contributing to the regional power grid through its run-of-the-river design. As a key component of the Snake River hydropower system, Lower Monumental Dam plays a significant role in energy production and river navigation in the Pacific Northwest.
History of Construction and Expansion
Construction of Lower Monumental Dam commenced in 1961, marking a significant phase in the development of the Snake River hydroelectric system in the northwest United States. The project was undertaken by the United States Army Corps of Engineers, who oversaw the creation of this concrete, run-of-the-river structure. Located on the lower Snake River in southeast Washington, the dam serves to bridge Franklin and Walla Walla counties. The strategic placement, situated six miles (10 km) south of Kahlotus and 43 miles (70 km) north of Walla Walla, was chosen to optimize water flow and power generation capabilities for the region.
The main structure and initial generators were completed in 1969, officially commissioning the facility. This initial phase established the core operational capacity of the hydroelectric powerplant. The engineering efforts focused on integrating the dam into the existing riverine landscape while ensuring structural integrity and efficient energy production. The 1969 commissioning date represents the primary milestone in the dam's early operational history, bringing the facility into service for regional power distribution.
Further expansion of the facility occurred in 1981 with the addition of further generators. This enhancement allowed for increased energy output and improved operational flexibility. The addition of these generators contributed to the plant's total installed capacity of 810 MW. The expansion reflected the growing energy demands of the region and the strategic importance of the Snake River system in the broader US energy infrastructure. The United States Army Corps of Engineers continued to manage the operational status, maintaining the facility as an active and vital component of the local power grid.
| Year | Event |
|---|---|
| 1961 | Construction of Lower Monumental Dam commenced. |
| 1969 | Main structure and initial generators completed; facility commissioned. |
| 1981 | Addition of further generators to expand capacity. |
Technical Specifications and Infrastructure
The structure bridges Franklin and Walla Walla counties, situated six miles (10 km) south of Kahlotus and 43 miles (70 km) north of Walla Walla. As an operational asset under the United States Army Corps of Engineers, the plant delivers a standard generating capacity of 810 MW. Under peak load conditions, the facility can achieve an overload capacity of 932 MW. These figures reflect the combined output of the turbine-generator units housed within the dam's concrete structure.
Spillway and Hydraulic Infrastructure
The dam’s spillway is designed to manage variable flow rates on the Snake River. The spillway spans a width of 572 feet (174 m). This dimension allows for controlled water release during high-flow periods, ensuring structural integrity and downstream navigation stability. The spillway utilizes a series of gates to regulate discharge, though the specific count and type of gates are part of the broader hydraulic management system overseen by the Corps of Engineers. The concrete construction of the dam provides durability against the continuous hydraulic pressure and sediment load characteristic of the lower Snake River.
| Parameter | Value |
|---|---|
| Entity Type | Hydroelectric Power Plant |
| Primary Source | Water (Run-of-the-River) |
| Operator | United States Army Corps of Engineers |
| Country | US |
| Location | Lower Snake River, Washington |
| Standard Capacity | 810 MW |
| Overload Capacity | 932 MW |
| Spillway Width | 572 feet (174 m) |
| Commissioned | 1969 |
| Status | Operational |
Navigation Lock and River Connectivity
Lower Monumental Dam incorporates a critical navigation lock that serves as a vital link in the inland waterway system of the Pacific Northwest. This infrastructure component is designed to facilitate the movement of barge traffic along the lower Snake River, effectively bridging the hydroelectric reservoirs that characterize this stretch of the river. The lock is a single-lift structure, engineered to handle the specific elevation changes required to transition vessels between adjacent bodies of water. Its dimensions are substantial, with a width of 86 ft (26 m) and a length of 666 ft (203 m), allowing for the passage of standard river barges and towboats that carry agricultural products, coal, and other commodities from the interior of Washington and Idaho to the Columbia River and beyond.
The primary function of this navigation lock is to connect Lake Herbert G. West, which lies upstream of the dam, with Lake Sacajawea, located downstream. This connectivity is essential for maintaining a continuous, navigable channel through the series of dams constructed on the Snake River. By linking these two lakes, the lock enables seamless transit for commercial and recreational vessels, reducing travel time and enhancing the efficiency of riverine transport. The integration of the lock within the concrete, run-of-the-river dam structure ensures that navigation and power generation can operate concurrently without significant interference. This dual-purpose design is characteristic of the United States Army Corps of Engineers' approach to river management in the region, balancing energy production with transportation needs.
The location of Lower Monumental Dam on the lower Snake River in southeast Washington places it in a strategic position for regional logistics. This geographic positioning underscores its importance to the local economies and the broader supply chains that depend on the Snake-Columbia river system. The navigation lock thus plays a pivotal role in sustaining the economic vitality of the area by ensuring reliable access to water-based transport routes. As part of the operational infrastructure managed by the United States Army Corps of Engineers, the lock continues to support the flow of goods and the connectivity of the river system, contributing to the overall functionality of the Lower Monumental hydroelectric facility.
Reservoirs and Regional Geography
Lower Monumental Dam functions as a critical node within the interconnected reservoir system of the lower Snake River in the northwest United States. The structure is situated in southeast Washington, bridging the boundary between Franklin and Walla Walla counties. Its precise geographic position is defined by its proximity to local landmarks, located six miles (10 km) south of the community of Kahlotus and 43 miles (70 km) north of the city of Walla Walla. This placement establishes the dam as a central feature in the regional hydroelectric and navigational infrastructure of the Pacific Northwest.
Upstream and Downstream Reservoir Connections
The hydroelectric operation of Lower Monumental Dam is intrinsically linked to the sequence of reservoirs that define the lower Snake River corridor. Upstream, the water body extends into Lake Herbert G. West. This reservoir stretches 28 miles (45 km) eastward, terminating at the Little Goose Dam. This continuous waterway facilitates both power generation and river navigation, connecting the operational zones of adjacent hydroelectric facilities.
Downstream, the river flows into Lake Sacajawea. This section of the reservoir runs 22 miles (35 km) in a southwest direction, extending from the Ice Harbor Dam. The configuration of these reservoirs creates a stepped hydroelectric system, where the run-of-the-river nature of the concrete dam at Lower Monumental helps regulate flow between these significant water bodies. The geographic span of Lake Herbert G. West and Lake Sacajawea underscores the extensive reach of the Snake River's damming projects, which transform the natural river gradient into a series of managed aquatic environments essential for regional energy production.
Why it matters
Lower Monumental Dam functions as a critical node within the extensive Columbia River Basin hydropower system, a network managed by the United States Army Corps of Engineers. As an operational hydroelectric powerplant with a capacity of 810 MW, it contributes significantly to the regional energy mix, providing consistent baseload power to the Pacific Northwest grid. The facility's design as a concrete, run-of-the-river dam on the lower Snake River allows for efficient energy generation while maintaining water flow continuity, a characteristic feature of the Corps' engineering approach in this basin. Commissioned in 1969, the dam has served as a stable source of renewable energy for decades, leveraging the natural gradient of the Snake River to drive turbine generation without the need for massive reservoir storage typical of upstream dams.
Navigation Infrastructure
Beyond its energy production, Lower Monumental Dam is integral to the inland waterway navigation system of the Columbia-Snake River Basin. The facility bridges Franklin and Walla Walla counties, creating a navigable link that connects agricultural and industrial centers in southeast Washington to the broader Pacific trade routes. The dam's lock system facilitates the movement of barges carrying grain, fertilizer, and other bulk commodities, reducing transportation costs for regional economies. Located six miles south of Kahlotus and 43 miles north of Walla Walla, the dam's strategic position helps manage water levels and flow rates, ensuring year-round navigability for vessels traveling between the lower Snake River and the main stem of the Columbia River.
Regional Impact
The combined hydroelectric and navigation functions of Lower Monumental Dam underscore its dual role in the regional infrastructure. By providing 810 MW of power, it supports local municipalities and industries, contributing to the economic stability of the surrounding areas. The dam's operation is coordinated with other Corps-managed facilities along the river, optimizing both power generation and water management. This integration highlights the strategic importance of Lower Monumental Dam within the larger Columbia River Basin system, where energy production and transportation infrastructure are closely linked to support the economic vitality of the northwest United States.
How does Lower Monumental Dam fit into the Snake River system?
As a concrete, run-of-the-river facility, its operational dynamics are defined by its position relative to upstream and downstream structures, particularly Little Goose Dam and Ice Harbor Dam. This specific configuration determines water flow characteristics, reservoir management, and the overall energy generation profile of the river segment located in southeast Washington.
Position within the Snake River Cascade
The dam is situated on the lower Snake River, bridging Franklin and Walla Walla counties. This positioning places it strategically between other major hydroelectric facilities. Understanding its role requires examining its relationship with Little Goose Dam, which lies upstream, and Ice Harbor Dam, which is positioned further downstream. The interaction between these structures creates a continuous chain of pools that supports both power generation and river navigation.
Run-of-the-River Operational Characteristics
As a run-of-the-river dam, Lower Monumental operates differently from storage-heavy reservoirs. The facility relies on the natural flow of the Snake River, modulated by the release schedules of upstream dams like Little Goose. This means that water levels and flow rates at Lower Monumental are directly influenced by the operational decisions made at preceding structures in the cascade. The concrete design supports this type of operation, allowing for efficient energy capture from the moving water without requiring massive seasonal storage. The 810 MW capacity of the plant reflects its ability to harness this continuous flow, contributing significantly to the regional grid managed by the United States Army Corps of Engineers.
The flow between reservoirs in this section of the Snake River is carefully managed to balance energy production with navigation needs. The proximity to Little Goose Dam means that water released from that upstream facility directly impacts the headwater levels at Lower Monumental. Similarly, the downstream connection to Ice Harbor Dam affects the tailwater conditions, which in turn influences the efficiency of the turbines at Lower Monumental. This interconnected system ensures that the hydroelectric potential of the lower Snake River is maximized while maintaining a navigable channel for commercial and recreational traffic.
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