Snake River – Lower Monumental Pool Area GRP

Table of Contents


Site Description

This plan covers the 29 mile reach of the Lower Monumental Pool Area (from the Little Goose Dam downstream to the Lower Monumental Dam), also known as Lake Herbert G. West within the Snake River.

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The Lower Monumental Pool Area is divided into 8 subregions: S-13, River Miles 69-74; S-12b, River Miles 67-70; S-12, River Miles 61-67; S-11b, Palouse River; S-11, River Miles 55-61; S-10, River Miles 51-56; S-9, River Miles 45-50; and S-8, River Miles 41-44.

Refer to Resources at Risk section for detailed resource information.
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Physical Features

The Lower Monumental Lock and Hydroelectric Dam Project is located approximately 41 miles upstream from the mouth of the Snake River at the west end of the pool area. Lower Monumental Pool (also known as Lake Herbert G. West) is 29 miles long and contains approximately 6,600 surface acres. The dam is 3,800 feet long at the crest, with a 335 foot long spillway (see map of dam).

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Mainstem, side channel, and island shorelines within the 8 subregions of the Lower Monumental Pool Area may include the following habitat types:

  • Exposed rocky headlands.
  • Wave-cut platforms.
  • Pocket beaches along exposed rocky shores.
  • Sand beaches.
  • Sand and gravel beaches.
  • Sand and cobble beaches.
  • Sheltered rocky shores.
  • Sheltered marshes.

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The Snake River originates in Yellowstone Park and travels approximately 1,000 miles west through Wyoming, Idaho, and Washington before finally emptying into the Columbia River at Pasco. The Snake River is the largest tributary to the Columbia River and is itself one of the major rivers in the United States.

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There is usually a perceptible current in both the Snake and Clearwater Rivers at the Lewiston-Clarkston area. Flow will have a perceptible affect on spill drift. Perceptible current will gradually disappear as a spill progresses downstream toward the next dam. As the spill travels downstream, the wind will begin to affect spill drift far more than the current.

It is nearly impossible to make a general rule-of-thumb to help predict wind behavior on the Snake River. The twists and turns of the canyon force the river to point toward, away, and crosswise to the wind. At any given instant, the wind can be calm in a sheltered stretch, blowing upstream in one place and downstream in another. In the immediate vicinity of the dam, there may be a perceptible current flowing toward the powerhouse, and/or the spillway.

The Snake River Darns are run-of-the-river projects. The Corps of Engineers North Pacific Division Reservoir Control Center (RCC) has regulatory control over river operations. Specific requests for changing flows or pool elevations must be directed to approved by the RCC. The darn operators do not have the authority to determine river/pool operation. They can, however, relay to RCC any public requests for special reservoir regulation. The best way for an Incident Commander or On Scene Coordinator to obtain immediate and accurate river flow and pool elevation data is to call the duty power plant operator at the darn(s).

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Climate and Winds

The climate of the region is temperate and moderate during most of the year. During the winter months, the onset of winter storms has been know to abruptly change conditions along the river from moderate to severe. Most of the annual precipitation occurs during the months of November through June. The average total annual precipitation is 12.43 inches.

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Throughout the year, wind gusting at high velocities can be expected in this area. Winds are generally from the east-southeast in the morning, shifting to the west in the afternoon. Wind, even a slight breeze, can have a big effect upon the movement of spills on the water. In the slack water behind the darn, the movement of the spill is almost entirely dependent upon the wind. Where the current is strong below the darn, both river current and wind will affect the drift.

Additional information may be available from the National Weather Service.

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Tides and Currents

As this GRP includes. the Lower Snake River area, there are no tidally influenced areas. Also, the Lower Monumental Pool Area has no free-flowing water, with water flow being governed strictly as a matter of when and how much water is allowed to pass through the spillways of the various darns.

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Nearly all flow into the Lower Monumental Pool comes from the Clearwater and Snake Rivers, after having passed through the Little Goose and Lower Granite Darns; the Palouse and Tucannon Rivers provide additional input. The low flows typically occur during the late summer, autumn, and winter months. The high flows occur during the spring snow melt. The upper reach of the reservoir is essentially a river and has strong runoff, especially during spring runoff. Nearer to the darn, the current is essentially nil, except for the area in front of the spillway and powerhouse. The area there may have very dangerous strong currents and undertows. River flows below the darn can vary from near zero to very fast and hazardous depending on: Flow into the reservoir; or, demand for electrical power.

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Risk Assessment

The Snake River, in conjunction with the Columbia River, is one of the principal environmental and economic resources found in the Pacific Northwest. Protection of this river is critical to the vast natural and cultural resources and populations which are dependent upon it.

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Native anadromous and resident fish species, including endangered sockeye salmon, depend on the Snake River and tributaries for their existence. Various species of waterfowl and other fauna are also dependent upon the Snake River. The waters of the Snake are used to irrigate crops and fill domestic, municipal, and industrial water needs.

Railroad/Barge Movements of Oils and Hazardous Substances: Union Pacific’s rail line runs along the southern and southeastern bank in the subject area. While this GRP is primarily concerned with responses to oil spills, basic information on hazardous substances movements through the region may also prove useful. This information is limited to basic emergency actions to take in response to an accidental chemical release.

The ten most abundantly transmitted hazardous substances on both rail lines includes:

  1. Chlorine CAS # 7782-5-5
  2. Sodium Hydroxide CAS # 1310-72-2
  3. Butane CAS # 106-97-8
  4. Propane CAS # 74-98-6
  5. Methyl Alcohol CAS # 108-11-2
  6. Asbestos CAS # 1332-21-4
  7. Anhydrous Ammonia CAS # 7664-41-7
  8. Phosphoric Acid # 7 664-3 8-2
  9. Ammonium Nitrate CAS # 7664-93-9
  10. Sulfuric Acid CAS # 7664-93-9

In addition to the movement of hazardous substances by rail, a vast amount of materials are also transported by waterborne vessels (primarily barges). Today, the navigable waters of the Snake are increasingly being used as a means of transport, especially tugs moving barges filled with commodities. Commodities are shipped up and down the river year round.

In addition to the products listed above, a partial listing of the petroleum products which move on the river includes:

  1. Asphalt
  2. Gasoline
  3. Jet Fuel
  4. Kerosene
  5. Diesels
  6. Crude Oil


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Resources at Risk


Because moisture is limited in much of the lands surrounding the project area, the waters provided by the Snake River provide an important part of the food, water and cover for numerous wildlife species. Wildlife that typically use the riparian and wetland areas associated with the project area include waterfowl, raptors, upland game birds, aquatic furbearers, and big game. Waterfowl, raptors, and aquatic furbearers warrant special concern in the event of an oil spill in this region.

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In an effort to improve habitat, the Corps of Engineers has established numerous Habitat Management Units (HMUs) along the Snake River. The size and complexity of these HMUs varies, but many of them include irrigation, tree and shrub plantings, food plots, nesting and brooding cover, brush piles, and nesting structures.

The HMUs established within the Lower Monumental Pool Area include (with river mile locations):

  • Megallon HMU – RM 45
  • No Name HMU – RM 45
  • Skookum HMU – RM 48
  • NoName2 HMU – RM 51
  • Ayer HMU – RM 51
  • 55-mile HMU – RM 55
  • Joso HMU – RM 57
  • Lyon’s Ferry HMU – RM 58
  • Tucannon HMU – RM 62
  • Texas Rapids HMU – RM 66
  • Riparia HMU – RM 67

Other significant wildlife areas, in addition to those habitats provided by HMUs, include shorelines with natural riparian vegetation, islands, wetlands, stream and river mouths (both free-flowing and impounded), and shallow backwater areas – especially those adjacent to natural shorelines.


Waterfowl are present in the Snake River dam pools throughout the year. Canada geese and mallard ducks constitute the bulk of locally nesting waterfowl. Availability of nesting and brood-rearing habitat are the most significant factors limiting the nesting productivity of this region. Natural nesting tends to be concentrated on islands rather than on the river banks. One notable exception to this generalization is the fact that many of the Canada geese in the upper Snake River pools nest on cliffs and ledges adjacent to the river. In some areas, nesting opportunities have been enhanced by providing artificial nest structures.

The greatest abundance and species diversity of waterfowl occur during those months when birds from other areas move into the region for overwintering. These include large numbers of Canada geese, as well as both dabbling ducks and diving ducks. These birds heavily utilize adjacent agricultural lands, lakes, marshes, backwater areas, and the Corps of Engineers HMUs for foraging and loafing.


The birds of prey most likely to be found in the immediate vicinity of the river include the prairie falcon, golden eagle, osprey, and balk eagle. Only the first two actually nest along the river. Because of their food and habitat preferences, however, these species are not likely to be at significant risk during an oil spill. Ospreys and bald eagles, the species that would be at greatest risk due to an oil spill, are generally uncommon along the Snake River except for migratory or transient individuals.

Aquatic Furbearers:

Aquatic furbearers occur in each dam pool. They include muskrat, beaver, river otter, and mink. In general, this group is dependent on riverine areas, embayments, ponds, tributaries, and riparian forests for den sites and foraging areas. The presence of a water barrier around den sites provides essential protection from predators, and is especially important when young are present in the early spring and summer.

Other Wildlife:

The project reservoirs provide essential habitat for numerous reptiles, amphibians, small mammals, bats, shorebirds, and songbirds. In general, riparian and wetland areas support higher population densities and species diversity than dryland shrub-steppe, talus, cliff, and/or grassland habitat, which are also prevalent along the project reservoirs. Habitats associated with the river generally support trees or dense grass/forb cover that provide more structurally complex areas and more abundant forage resources than adjacent uplands.

Threatened and Endangered Species: 

Of the wildlife species likely to be found along the Snake River in this region, only the bald eagle is federally listed as a threatened species. It is anticipated that bald eagles will be downlisted in the near future.


The Snake River is used for rearing and transportation by many fish stock groups. A brief description of these fish groups can be found on the following pages; see salmonid migration chart.

The focus of response in the event of a spill of oil or hazardous materials into the Snake River should be the protection of the juvenile populations and the food web that supports them. Juvenile fish rear and feed in a shallow water environment, and are not sufficiently mobile to escape the effects of oil. The major food source for all juvenile fish is also located in this environment. Destroying this habitat can have a devastating effect on the survival of juvenile populations thereby impacting the survival of the total fish population and ultimately other organisms that depend on these fish for food. Oil spill response strategies should include priority protection of shallow water habitat.

Coho (Silver) Salmon:

Adult Coho enter their spawning areas starting in late August and lasting until December. The major migration occurs from August to mid September. Rearing takes place in smaller tributaries. Juvenile Coho spend about a year in the stream they were spawned, feeding mainly on zooplankton and emerging insects. Migration downriver generally occurs from April to June, with the juveniles utilizing shoreline cover and open waters.

Chinook (King) Salmon (Threatened Species):

Spring chinook:
Adult spring chinook begin entering the Columbia River in February and reach the Snake River by April. The peak migration occurs from April through June. Spawning occurs in many of the Snake River tributaries. Young chinook feed on aquatic insect larvae, terrestrial insects, and small invertebrates. Juveniles
outmigrate/rear as yearlings from April through May, and utilize a deep water environment and are dependent upon benthic prey.

Summer chinook:
Adult summer chinook begin entering the Columbia River in May and reach the Snake River by June. The peak migration occurs from June through August. Spawning occurs in many of the Snake River tributaries. Young chinook feed on aquatic insect larvae, terrestrial insects, and small invertebrates. Juveniles outmigrate/rear as yearlings from April through May. Some fingerlings from the spring and summer runs may stay in the river up to 1 1/2 years before migrating to the ocean.

Fall chinook:
Adult fall chinook begin entering the Columbia River in July and reach the Snake River by August. The peak migration occurs from August through October. There are two basic races of fall chinook – tules and upriver-­brights. Tules spawn in September, and generally outmigrate in the spring. Upriver-brights are a late spawning, November through January, upriver variety including hatchery and wild fish. Wild stock rear in shallow water rapids within the dam reservoir pools.

Sockeye Salmon (Endangered Species):
Adult sockeye begin entering the Columbia River in April and reach the Snake River by May. The peak
migration occurs from June through August. All sockeye are wild stock, and require spawning grounds in streams lying adjacent to lakes. After the eggs hatch, juveniles migrate to a lake and spend I to 3 years there before they outmigrate to the ocean. Outmigration generally occurs in May and June.

Steelhead Trout:
Steelhead can be found in the Columbia and Snake Rivers year round. There are two runs of steelhead, summer and winter. Summer steelhead begin entering the Columbia River in February and reach the Snake River by April, with the peak migration occurring from June through October. Summer steelhead spend the winter in the Columbia and Snake until they move into their home streams to spawn in the spring. Winter steelhead migration begins in November and continues through April. Juvenile steelhead generally outmigrate in March through June.

Other Resident Fish:
Other resident fish can be found in the waters of the Snake River year round. These fish rear in slower side water pools where there is more cover and a slower water flow rate. Juveniles would be most vulnerable to the effects of an oil spill.

Shoreline Types and Sensitivity:

The type of shoreline, degree of exposure to waves and currents, and biological sensitivity are the main criteria for selecting appropriate treatment techniques. Each shoreline type has particular properties (including vegetation types) which facilitate or resist the penetration and persistence of oil. Areas of comparatively uniform sediment type and grain size experience a deeper penetration of oil.

Grain size definitions are:

  • Mud <0.0625 mm
  • Fine Sand 0.0625-2mm
  • Medium to Coarse Sand 2-4mm
  • Pebble/Cobble 4- 256 mm

Persistence of oil in a particular area is directly related to the intensity of wave action, tides, and currents. Based on numerous oil spill studies of shoreline characteristics, treatment, and oil impact, the matrices in Chapter 6 were formulated following the basic prototype of the Environmental Sensitivity Index Atlas.

The environmental sensitivity index (ESI) system ranks coastal environments on a scale of 1-10 or 11 (less sensitive to more sensitive) with respect to oil spill sensitivity and potential biological injury is being used for mapping extensive areas of the coastline of the U.S. Generally speaking, areas exposed to high levels of physical energy, such as wave action and tidal currents, rank low on the scale while sheltered areas have the highest ranking. The shoreline types used in this manual are a combination of the two similar systems used for the Delaware/Pennsylvania/New Jersey ESI Atlas, and the Maryland and Virginia atlases. The numbering system for the Countermeasure Manual Shoreline Types does not correspond exactly to either atlas; however, the corresponding shoreline types can be identified easily from the ESI maps and reassigned the appropriate number (after field verification.) The shoreline ranking system provides a useful first step in the design of contingency plans because it identifies the priority areas that require maximum effort for protection and cleanup. Strike teams and contractors with this document can focus their activities · on environmental priorities, particularly during the first few hours and days of the spill.


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Economic Resources at Risk – Summary

Because GRPs focus only on protection of public resources, the numerous private water intakes along the Snake River are not listed in this plan. Public recreation/habitat areas with water intakes in this pool include:

  • Lyons Ferry State Park.
  • Lyons Ferry Marina/Port of Columbia.
  • Fifty-five Mile Habitat Management Unit.
  • Skookum Habitat Management Unit.

These sites are also identified under the “Resources Protected” sections of applicable strategy matrices in Section 4.3.

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General Information

Flight Restriction Zones: Flight restriction zones have been designated in the GRP to minimize disturbance to certain wildlife species. An identified location could represent a heron colony or the individual nest of a sensitive species such as bald eagle. While some zones may be restricted year around, others will be in effect only during months listed in the matrix. The no-fly bubble is the area within a 1,500 foot radius and below 1,000 feet in altitude around the location.

All aircraft, including those from the government, contractors or media, are expected to avoid these zones when restrictions are in effect. In the event that one of these zones must be entered during a spill response in the Lower Monumental Pool, clearance must be obtained from the Washington Department of Fish and Wildlife (WDF&W) and the United States Fish and Wildlife Service (USFWS. During oil spills, pilots are also asked to avoid disturbing any large concentrations of birds and other wildlife. By keeping a safe distance or altitude, pilots can prevent the accidental hazing of unaffected wildlife into oiled areas and minimize the risk of aircraft/ bird collisions.

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Hazing: Hazing or directed harassment, is a method used to drive or herd wildlife out of an area where they are at risk of becoming oiled. Hazing techniques include the use of visual and audio devices, personnel for herding, vessels and aircraft. In the right circumstances it can be an effective tool for protecting some wildlife species. In other cases it can be disastrous as unaffected wildlife can be driven into oiled areas, or forced to abandon nests or young.

Before hazing can begin for all species of wildlife in the Lower Monumental Pool, clearance must be obtained from the Washington Department of Fisheries and Wildlife and the United States Fish and Wildlife Service. All hazing efforts during a spill will be directed by these agencies. The deliberate harassment of wildlife without first securing permission from these agencies is a violation of Federal and State laws.

The following information must be provided for a determination on whether hazing might be authorized in a given situation:

  1. Description of the situation where hazing authorization is being sought.
  2. Location to be hazed.
  3. Species of wildlife to be hazed and number of animals.
  4. Methods and equipment used.
  5. Date and time of hazing.
  6. Name, phone number, radio frequency, pager number and the amount of hazing experience of the individual requesting permission.

The responsible agencies will evaluate each request on a case by case basis. All hazing of marine mammals, threatened and endangered species, and all hazing by aircraft will be performed only under authority and general supervision of WDF&W, U.S. Fish and Wildlife Service, National Marine Fisheries Service, or persons designated by these agencies. Representatives of these agencies can be contacted through the planning section of the Unified Command System during the spill event.

Oil Countermeasure Matrix: The Northwest Area Committee has developed a manual and a series of matrices as a tool for shoreline countermeasure response. The shoreline countermeasures matrices and manual can be found in the main body of the Northwest Area Contingency Plan.

Shoreline countermeasures following an oil spill are a critical element in determining the ultimate environmental impact and cost resulting from a spill. Local response organizations and agencies have developed mechanisms for identifying shorelines requiring treatment, establishing treatment priorities, monitoring the effectiveness and impacts of treatment, and for resolving problems as the treatment progresses.

Each section of the manual has been adapted to the specific environments, priorities, and treatment methods appropriate to the planning area. These elements provide the information needed to select cleanup methods for specific combinations of shoreline and oil types. Local information on shoreline types (Discussed in Resources at Risk) can be obtained from Environmental Sensitivity Index (ESI) atlases prepared by NOAA for northern and southern Puget Sound, the Washington & Oregon coast, and the Columbia River. At this time, shoreline information for the Columbia River offers the closest analogy for shoreline cleanup questions on the Snake River.

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