|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
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.
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
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
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.
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.
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.
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
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.
you think of any positive or negative attributes for plants from a soil of
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.
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)
Large cup or other container
Watch with second hand or stopwatch
Select a clear tube and cover one end with nylon mesh. Use tape to
secure the mesh to the tube.
- Make a hypothesis about how quickly you think the water will flow
through the soil.
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.
When making your hypothesis consider your parameters of when to start
and stop timing, i.e.,
- How would different timing parameters affect your analysis?
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.
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
Use a pH strip to check
the acidity of the effluent in the cup.
- Compare your pH reading with the soil survey data.
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
- 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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