Glacial Geology and Lake Missoula Floods

Soils and Aquifers: Spirit Lake giant current ripple field.
Soil Infiltration Activity
NEW Workshop Images


Today we will look at some of the field evidence for a geologic cataclysm debated by North American geologists from the 1920's to the 1950's. The origin of the Channeled Scabland and associated features in Washington, Oregon, Montana, and Idaho was the basis of this long-lived debate between Uniformitarianists and Catastrophists in geology. The debate was eventually settled with the recognition of a huge water source in glacial Lake Missoula that was impounded by an unstable Pleistocene ice dam. We now credit a number of catastrophic events for major roles in geology history and appreciate that rates of geologic processes are not uniform. 

Farragut State Park is located where the Lake Missoula Floods broke out from the end of Lake Pend Oreille. We will look at the part of the ice-age floods story just downstream of the ice dam. Many of these large-scale flood features are best recognized from aerial photography and satellite imagery. In fact, the sites you will visit today were used by NASA to design the Pathfinder Mission for exploration of large-scale flooding on Mars. We will use field observations, topographic maps, and aerial photographs to interpret these extraordinary landforms.

The ice lobe that blocked the Clark Fork river also filled the Pend Oreille basin. At its highest level, Lake Missoula rose to an elevation of 4,100 feet against the ice front. The surface elevation of Lake Pend Oreille is now 2,063 feet. Failure of the ice dam released as much as 500 cubic miles of water from Lake Missoula. Upon the breakout from Pend Oreille basin the flood inundated Rathdrum Prairie with water hundreds of feet deep. The rate of flow is estimated to be 10 times the present flow of all the world's rivers combined. The water was directed across the Purcell trench toward Spirit Lake and built a huge gravel bar covered with giant current ripples. The flood gravels from the Rathdrum aquifer are an important water source for nearly 400,000 people in Idaho and Washington as well as a vital source for construction materials.


Soils and Aquifers - Spirit Lake Giant Current Ripple Field

We will be performing a number of field exercises to gain a better under-standing of the hydrological interactions between the surficial geology of the Rathdrum Prairie and the Rathdrum Prairie-Spokane Valley Aquifer. We will investigate the similarities and differences between the Kootenai, Bonner, Rathdrum and Mokins soils and why they are important to the aquifer. We will identify and measure soil horizons and see how quickly water infiltrates defined horizons. Soil characteristics such as depth, slope, texture, drainage, permeability, and organic matter content are important in order to understand the soil’s ability to store nutrients and filter potential contaminants.

The Rathdrum Prairie-Spokane Valley Aquifer is located primarily in Kootenai County, Idaho and Spokane County, Washington, and is recognized as one of the most productive in the country. The water in this unconfined aquifer occurs in the spaces between the grains of coarse sand, gravel, cobbles, and boulders deposited during catastrophic glacial outburst floods of Pleistocene Glacial Lake Missoula. The Rathdrum Prairie-Spokane Valley Aquifer is the only significant source of good-quality water supply in the area. In 1978 the U.S. Environmental Protection Agency designated the aquifer as a “Sole Source Aquifer” for drinking water. In 1980 the Idaho Department of Health and Welfare declared the aquifer a “Special Resource Water.”

The deep and well-drained Kootenai, Bonner, and Rathdrum Soils formed in the glacial sands and gravels deposited by Glacial Lake Missoula Floodwaters and are capped with a discontinuous layer of volcanic ash and loess. Owing to the high permeability of these soils and the flood gravels, little protection exists for the groundwater from surface land use activities.

A contrasting soil to the Kootenai, Bonner, and Rathdrum soils is the Mokins Series. Mokins soils formed in a thin mantle of loess and volcanic ash over Miocene sediments. These soils can be found above the Miocene Basalt that rims the Rathdrum Prairie.  Mokins soils include a highly leached buried soil, a paleosol, of white silt loam. Below this paleosol (18-20 inches), multiple horizons of distinct reddish yellow iron mottled clay (plinthite) are evident. Modern plinthite soils occur in the piedmont of Georgia and South Carolina. The Idaho plinthite formed in the Miocene when the climate was warm and wet, like in the southeast U.S. today. The clay in the Mokins soils restricts any downward movement of water.

In Idaho, water quality enforcement generally fall into six categories: leaking underground storage tanks, violations of drinking water standards, surface water discharges and spills, mining, engineering (i.e., sewer construction) and groundwater.

Measuring how fast water infiltrates a soil will provide an understanding of the factors that control the soil’s ability to accept, hold, and filter water. What happens to water when it passes through soil depends on many things such as the size of soil particles (texture and particle size), how the particles are arranged (structure), how tightly they are packed together (bulk density), and the attraction between the soil particles and the water.

Observing and measuring these characteristics in the field will help us better understand a soil’s ability or inability to regulate potential threats to our drinking water supplies.

1.Some soils absorb water relatively quickly then hold the water like a sponge.

*Can you think of any positive or negative attributes for plants from a soil of this type?

2. Some soils allow water to move through to the subsoil quickly, while others keep water from getting in at all.

     *Would there be a threat to groundwater quality from soils of this type?

3. How would YOU categorize the Kootenai, Bonner, Rathdrum, and Mokins soils?

To help us categorize these soils we will characterize and measure soil profiles following the activity “How to Observe and Record Soil Properties”. To help us determine the rate water infiltrates our defined horizons we will follow the “soil Infiltration” activity.

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Clear plastic tube or bottle (ends removed) – one for each individual or pair.
Piece of nylon mesh screen
Length of tape
Spoon or trowel (do not use your hands)
Water supply (distilled water)
Graduated cylinder 
Large cup or other container
Watch with second hand or stopwatch
pH strips


Select a clear tube and cover one end with nylon mesh. Use tape to secure the mesh to the tube.
Using something other than your hand, fill the tube about half full with a good representation of the soil horizon you’re sampling. You may need to remove the larger cobbles.

- Make a hypothesis about how quickly you think the water will flow through the soil.

When making your hypothesis consider your parameters of when to start and stop timing, i.e., 
    Start: As the water first touches the soil or after all the water has been poured into the tube.
    Stop: As the first drip appears or all of the water comes out the bottom.

- How would different timing parameters affect your analysis?

Holding the tube over a large container to catch the effluent, use the graduated cylinder to pour a measured amount of distilled water on top of the soil. Be careful not to allow water to overflow into the container.

- Did your hypothesis hold water?  Why or why not?
- What happens when you run the test again with the same soil?
- In your discussion include the concepts of porosity, permeability, and particle size.

Use a pH strip to check the acidity of the effluent in the cup.

- Compare your pH reading with the soil survey data.

Contrast the measurements of other soil samples by repeating the above steps for different horizons or from different locations.

- Did you find any differences of infiltration in any one profile?
- Was there one horizon with a higher/lower rate than other horizons?
- Was there a rate change in samples taken from the top and bottom of one horizon?
- Did you find any differences of infiltration between the soil in the trough and the soil on the crest?
- How does knowing the infiltration rate of a soil help us understand the processes involved in recharge and filtering out contaminates before they reach the groundwater?
- How could we increase the infiltration rate of a poorly drained soil like the Mokins, so it could be used for septic drain-fields?
- What could we do to decrease the chance of groundwater contamination where there are well-drained soils like the Kootenai?

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Workshop Images

Gravel ash site.jpg (178227 bytes)
Measuring and sampling a bed of ash from Mount Mazama's cataclysmic eruption ~7000 years ago.

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A morning spent orienting aerial photography and topographic maps to prepare for a day in the field.

Landform Trek1.jpg (136184 bytes)
Armed with a GPS, topographic maps, aerial photographs, and inquiring minds 
a group of teachers set out for a day of field investigations.

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Along with morning coffee, teachers listen to Roy's description of the big picture for the day's activities.